For the fraction of a percent of readers here who never poked their heads into the usenet group alt.folklore.urban, the glass flow trope worked as a bit of a shibboleth there for some years.

Oddly enough, I had never heard that particularly scientific legend, at least, if I had, I hadn't remembered it. It doesn't actually make any sense when you think about it. Solids generally don't become liquids or liquid-like unless conditions change so as to make them unstable. But glass is appealing, magical stuff, especially in mirrors: it stimulates the imagination delightfully, particularly if one has seen it made. I work with glass in the form of glaze (as in pottery & scultpure). Part of the fun of the process is not knowing exactly what will happen when a piece is glazed.

Lizzy, I've done the same. Glass is charmingly perverse stuff.

Now I want to make a window pane out of silly putty.

Is the link at the asterisk supposed to be the same as the link at "tapered"?

I first read this in a Heinlein book, I think. You are shattering my cherished illusion that RAH was always scientificly rigorous. (Although the truth is fascinating. Who knew glass was such interesting stuff?)

While I was recently cured of this particular fable, I still believed that glass does flow, but much too slowly to be noticeable even over hundreds of years; thus flow occurs but does not account for the thick-at-the-bottom panes in those old churches. (And not only does the glass never overflow the paning at the bottom, it never gets "melt-holes" in the top, either, no matter how old it gets.)

But the subtle second-order phase transition means that's wrong too.

One of the coolest things I've heard about recently is the creation of metallic glasses. Stronger for their weight than crystalline metal. Of course, I immediately started wondering about their musical properties...if they differ at all from crystalline metal.

I do my best to shoot down this myth wherever I find it.

Another is the use of Bernoulli's principle to explain airplane lift.

So, you're, ummmm, saying wood doesn't flow?

Um...Mark? Explain how Bernoulli's principle doesn't explain airplane lift? I still believe(d) that one. How does it work then? I just saw a Nova program (or something) a few months back that explained it in Bernoulli terms.

I HATE it when I believe nonsense, especially when I've been telling it to other people!

As somebody who gets to tell people cool stuff about materials for a living, my thanks go out to Our Gracious Hostess for helping to dispel this myth. Ashby and Jones point out that it would take on the order of 10 000 years for glass to flow appreciably at ambient temperatures; we can come back and check out our cathedrals (or shopping malls, or junked cars) in a few millennia.

They also mention that there is one important ceramic, with a relatively low melting point, for which flow under load is really important - ice, in the form of glaciers.

I don't know... While I'm willing to concede that it probably is a myth, I don't think you've provided evidence to that end.

The manner in which small quantities of window glass (or for that matter, sheet glass of any kind) was made in the pre-industrial/early-industrial period was by getting a blob of molten glass onto your punty (the blowing rod), and blowing out a bubble which was then flattened into a plate. This plate was then heated and spun, using the blowpipe as an axle, forcing the plate out wider & wider. When you were finished, you snipped the plate (up to 1.25m across, or larger) from the punty, annealed it to remove the stresses, and then scored it into sheets. (The punty mark at the middle of the plate is what made 'bullseye' glass.)

Early industrial-era glass was made by floating molten glass on a bed of molten tin. You then ran a heated steel roller over the molten glass, forcing it down into a more-or-less even layer. This is called 'float' or 'water' glass, and the process results in the wonderfully ripply, vaguely watery appearance of pretty much all glass produced until the early- to mid-20th century, when techniques for manufacturing plate glass as we know it today came into widespread use (our house dates to 1914, and has some original float glass windows; one of them has a central pane which is almost 1.5m across and probably almost 1m tall).

Neither the hand-blown technique nor the float glass method (the float glass method even less so) result in (many) panes of glass that are shaped as you describe. Glass wants to achieve the same level when heated - it really, really, really wants to be no more than 5mm thick or so, and will spread out on its own to a uniform level with astonishingly little persuasion. You have to coax it to get thinner than 2-3mm, but not all that much...

Glass doesn't melt like other solids - so many other substances suddenly give way from one state to another. Glass gradually phases, more like sugar candy softening under gentle heat than, say, an ice cube. I could see how observing glass under heat could lead one to believe that it is more of a funky liquid than a 'true' solid.

Glass is way cool stuff. I dabble in soft glass kiln-forming and beadmaking - but I've never blown glass.

I first read this in a Heinlein book, I think. You are shattering my cherished illusion that RAH was always scientificly rigorous.

How Airplanes Fly: "The second description we will call the Popular Explanation which is based on the Bernoulli principle. The primary advantage of this description is that it is easy to understand and has been taught for many years. Because of its simplicity, it is used to describe lift in most flight training manuals. The major disadvantage is that it relies on the 'principle of equal transit times' which is wrong. This description focuses on the shape of the wing and prevents one from understanding such important phenomena as inverted flight, power, ground effect, and the dependence of lift on the angle of attack of the wing."

D'oh - lost a couple of sentences in formulating my comment - so, what I meant to include was something to the effect that any irregularities are due to the manufacturing processes used; any instances where the glass is thicker on the bottom than on the top is probably because in those cases, the glass was noticeably thicker on one end than the other. If you were to perform a comprehensive survey of old glass panes, I'm guessing that you'd find that many of them are thicker on one end than the other - but that it takes a certain 'obviousness' threshold for the thicker end to wind up on the bottom. This would also contribute to the myth: the thicker-bottomed panes are, by definition, somewhat more remarkable.

Thus ends our first SWAG of the day.

Neither the hand-blown technique nor the float glass method (the float glass method even less so) result in (many) panes of glass that are shaped as you describe.

But... but... but I was told that myth by a glass-blower! *sobs* In Grants Pass! On a First Friday Art Walk when the open up the forge to spectators! *wails*

Re: Bernoulli's principal - [puts on pilot's headset] I'm pretty sure I remember being taught that that's what causes the wind flowing over the top of a mountain to be really really bad for the structural integrity of a small plane. And also what causes carburator icing.

Protected static:

Two things: one, all of the pieces of glass cut from the 1.25m-diameter spun glass, except the bullseye, would be thicker on the side closer to the axis (the blowpipe). That's why older glass has a thick side and a thin side.

Second, I think you've conflated 'hot-rolled' glass (made by flattening a high-viscosity sheet of glass between rollers) and float glass, made by floating low-viscosity glass on tin. The latter is the way that sheet glass (large panes with parallel sides and excellent optical clarity) is currently made.

Oh, and Xopher - I have a sample of amorphous metal in my lab - I'll have to go whack on it when I head back. My guess is that it should have markedly different musical properties than the traditionally-used metals since it has an unusual composition and is particularly hard. But I know that assessing the acoustic properties of materials for instruments is insanely complicated, and I couldn't presume to address that side of things.

debcha: Yeah, the hand-blown method would produce pieces with an integral slope, but probably not quite as many as you'd think. It would, however, produce enough obviously sloped pieces to contribute to the myth.

As to the 2 processes, it is quite possible that I have conflated them to a degree, though my understanding (derived largely from reading the semi-technical materials from glass suppliers - although the materials are written for non-technical audiences, and there's no guarantee that they don't contain myths or radical oversimplifications as well...) was that the float method could also produce rippled and wavy glass.

Protected Static:
The punty is a solid rod, as distinguished from the hollow blowpipe.

The method I learned involved blowing the "light bulb", then attaching a punty to the other side and cutting the pipe loose. Spinning/heating makes first a bowl shape as the hole from the pipe expands, ultimately a flat disc with the bullseye from the punty in the middle. There is frequently more thickness at the outer edge of the disc, though it depends on how often you reheat the blob during the bubble phase, and how evenly you blow out the bubble. Elongated bowls with deliberately thicker edges can be made by holding the punty vertical and letting the shape sag towards the floor.

Protected static, as I understand it, glass panes were cut down from blown disks and cylinders until the 1870s. After that, glass was poured flat, rolled, and then ground and polished. Plate glass, with its genuinely regular surface, was a premium product. Less premium products were less regular. The technology for drawing continuous sheets of glass was developed in the Teens, so your 1913 house would most probably have poured-and-rolled glass in its windows. True molten-metal float glass didn't come in until 1959.

Unless, of course, glassmakers use more than one sense of the word "float."

Dan, crown glass -- that's the round stuff -- is thickest in the middle and thins further out.

Wood floors tend to be perfectly flat, which is exactly what you'd expect from the liquid wood theory.

See, that's what I like about this site - I almost always leave feeling smarter than when I started (after I get over the feeling dumber part, that is...). It has a wonderful way of highlighting the gaps.

Mind the gap ;-)

I see places where liquid wood has flowed around chain-link fences all the time.

TNH: To specifically respond to your question about more than one sense of 'float', I dunno. I've seen it applied to modern plate glass as well as stuff that was decidely un-modern looking. Could be miscomprehension on my part, could be sloppiness on their part. Could be a modern process that deliberately undoes some of uniformity.

I'll take miscomprehension for $200, Alex.

Carol K: Thanks for the clarification - I was unaware of the distinction.

Falsback to highscool science class and the demo for glass sheets without a barrier between them sticking together over time. Fine at the start of the semester but by final exam, pain in the ass to pry apart.

Float glass was introduced by Pilkington in the late fifties; it's a very interesting process.

Sun-blued (or even violetted) glass is a result of electrons getting kicked out of iron impurities in the glass by high-energy photons, and then (because glass is a very viscous liquid) not being able to migrate back to the original atom site. Houseowners on Beacon Hill in Boston are proud of this effect because it attests to the antiquity of the property. Newer glass doesn't do this, though; additives such as manganese cancel out the iron.

There's an accurate explanation of how flight works in _Nature's Flyers : Birds, Insects, and the Biomechanics of Flight_. However, I find it a little too involved to summarize here, especially since the book is at home and I am not. (If I had a better technical understanding I'd probably be able to explain it more simply. However, I didn't take fluid dynamics in college. The closest I came was electromagnetic theory.) Also, I, coincidentally, just finished that chapter this weekend so I should probably read it again before even attempting an explanation.

However, if you're interested, the book, among other things, debunks the use of Bernoulli's principle to explain lift generated by wings and gives a scientifically sound explanation for the phenomenon.

This is one of my favorite bits of scientific misinformation, and a great example for pointing out the need to be skeptical about authoritative-sounding claims. In my experience, there's one surefire way disprove it in a sentence or so. Just observe that, if glass flowed quick enough to distort by millimeters over the course of centuries, those who make glass telescope lenses, which must be smooth and accurate to the scale of tens of nanometers, would find their work sagging into uselessness almost before it could be installed.

My understanding is that glass is amorphous - the atoms don't line up in regular patterns like they do in crystals. This makes it like a liquid, but doesn't make it a liquid. Or we'd be having very different experiences with glass jars holding instant tea.

I suspect that the glass-panes-sticking-together has as much to do with moisture and microscopic grunge sticking the pieces together as anything else.

Glass discoloring over time got mentioned many years ago in, IIRC, National Geographic: they were talking about 'desert glass', where it's been out in the sun for years, and has changed color.

I don't know how aeroplanes fly, although I have had it explained to me repeatedly. This is largely because I have also had it explained to me repeatedly (by my Dad, who worked for 35 years as an aeronautical engineer) that the explanation I can understand - based on Bernoulli - is wrong.

I don't know if this leaves me any further along than when I started. Mind you, I had been happily accepting the flowing-glass factoid without ever thinking too hard about it, so I'm glad I came by.

if the windows found in early Colonial American homes were thicker at the bottom than the top because of "flow" then the glass found in Egyptian Tombs should be a puddle.

Heh, great myth killer there.
Also, I suddenly have a desire for a "puddle of glass"...

I recall some short story from a year or two back about a guy who carved marble and his wife died and he blames the mountain for it (she fell? can't remember.) anyway, the mountain responds by having his marble slowly puddle. The story starts with a statue he just finished carving, and bits and pieces slowly "melt" and the guy is trying to figure out what the heck is going on, and by the end, there is a puddle of marble on his floor.

Helicopters don't fly.
They beat the air into submission.

Dan, crown glass -- that's the round stuff -- is thickest in the middle and thins further out.

I always am surprised by why the Bernouilli-based explanation is regarded as easy to understand. It's not intuitive at all for me that faster-moving air should have lower pressure -- if anything, it looks to me like the curved upper wing surface might squeeze the air above it and drive the wing *down*. That's not what happens, but it seems more intuitively plausible than "fast-moving air sucks the plane up".

The cheap and almost-correct explanation based on angle-of-attack seems far more natural to me. Wings are basically ducts, canted downwards. When I put my hand out a car window at an angle to the ground striking the bottom of my hand and pushing my arm up. That's basically what wings do, except (this is the "almost-correct" part) that it turns out in practice that air bending over the top of the wing and down is the more important part of the flow. But the point is that wings drive the air down, and the plane stays up.

I just wrote "I always am surprised by why...", but meant "I always am surprised that..."

Well, nuts.

Here I am standing in this house, and I was sure I'd be okay if I threw the stone very slowly.

I don't know... While I'm willing to concede that it probably is a myth, I don't think you've provided evidence to that end.

Here you go.

NASA has a great stance on the situation which seems to be "Yeah, well... *sigh* ghod, this is going to take forever. Okay, so people talking about Bernoulli aren't exactly right, but they aren't exactly wrong either. [sotto]why did I get myself into this...[/sotto] Let's just say it's way more complicated".

My grandfather was a potter by trade, and mixed his own glazes; I didn't learn until after he died that he was esteemed in some circles for the science of his glazemaking almost as much as for his artwork. This thread's making me wish I'd had the patience, years ago, to learn some of that strange alchemy from him.

Also, thanks to Xopher, I now have a new toy on my wishlist: a transparasteel lute.

Dan...I hate to break it to you, but while metallic glasses have many properties associated with glass, transparency isn't one of them. Silica glass is transparent because silica is (or can be). The metallic glass they showed on the TV show I saw was all shiny but quite opaque.

Party pooper. Next you'll be telling me I can't have a flying car, either.

Arg. Air traffic controllers are still taught that Bernoulli explains flight. I was deeply offended to learn, quite recently, that this is incorrect. I cannot recall the exact true mechanism; it is complicated enough to not be a strong competitor with Bernoulli.

Xopher: You mean I can't have that transparent-aluminum aquarium, either?

Xopher, transparent aluminate glass was created in 2004:

Glass Breakthrough at 3M

The article has a good explanation for why it's so easy to make glass from silica and so difficult to make from other materials.

I thought it was manganese or magnesium, one of those M elements, that turned glass purple. But I'm remembering bits from the Seattle Underground tour I took years ago.

On glass flow, I was told earlier this year, while I learned to break knives for microtome sectioning (so much fun!), that the knives will dull themselves over time. They're as sharp as they'll ever be right as you break them. I assumed it was a weirdness of glass, but didn't connect it to the glass-flow bit. I have heard that knives will do much the same thing, returning to a duller or sharper shape, but I haven't observed it myself.

It's an important trope in Walking on Glass by Iain Banks (or is it Iain M. Banks; always get confused between the two). One of the protagonists, trapped in a castle, discovers that the glass in the windows has begun to bulge out at the bottom of the frames, demonstrating how long the castle has been in existence or something.

IIRC the quick way to discredit the Bernoulli effect on the carefully shaped airfoil as the only thing holding a plane in the sky is to fly upside down. A quick way to show that the Bernoulli effect is somewhat involved is to try and takeoff with a wee bit of frost on the airfoil, spoiling the laminar flow of air over the wing.
If you look at the panes of really old glass that were thicker at the bottom, the edge should still be fairly sharp (but I don't know if the glass would have been annealed after cutting back then-- which might soften the edge a little bit).
Congealed tar can be liquid enough to flow over long periods of time as this IgNoble winning experiment shows .

Bah. Of course glass doesn't flow anymore, it's now made by boring people. It was much more fun when you could only get glass from mediterranean or arabic people... then it was flowing like water, creating shapes of exhilarating irregularity, and each time the artisans blowed into their pipes, you could get a cursed wine glass to poison your enemy, a fragile bottle to capture foolish gods, or a decorated window to see the future through.

(Then you could buy these wonders and they'd magically become worthless antiques, and very opaque in several areas, but of course that was not the point. Jeez, who'd want glass that doesn't magically change?)

Giacomo--

Yep. See A.S Byatt's "The Djinn in the Nightingale's Eye."

glass is a supercooled liquid,* or an extremely viscous liquid

Why does the asterisk point to a website about wooden shingles? I couldn't find a reference to supercooled liquid anywhere on that page.

Dan Layman-Kennedy: Flying car? I was hoping for a flying carpet.

transparent aluminate glass was created in 2004:

Oh no! That means that when Kirk, Spock, and the crew comes back to get some humpback whales, Scotty won't be able to barter a secret formula for transparent aluminum for six inch stock of plexiglass. Maybe we should start a fund, have a bake sale.

Giacomo, can I come visit your universe? Sounds like my kind of place exactly.

I'd just like to express my admiration for the word "flooped."

Thank you.

Cassie:

I believe that microtome blades made of broken glass get dull because of chemical attack from the atmosphere - the sharp edge would be highly susceptible because it's almost atomically sharp (IIRC) and those atoms are hanging out there naked - they'd really rather be huddled inside with their neighbours or joined up with one of the O2 or other molecules floating around. In addition, the freshly-broken surface will have some cracks, which will grow, and will certainly accumulate cracks when you brush against them, set them down, etc. so that will take off the edge.

Actually, come to think of it, freshly-broken microtome blades, in use, may be one of the few cases where the glass flows to a practically-important degree under load. Glass under load can either break due to cracks or it can (in the absence of cracks, and under high enough load) deform, the same way that metals do. Obviously, in most situations, the breakage due to cracks wins handily over the deformation. But in this case, since the edge of the glass blade is so thin, it's possible that the stress (force divided by area) is high enough that it actually flows. I'd have to get some numbers to check that as a possibility, though.

This is an awesome book that talks about how cool freshly-drawn or broken glass is; it's one of the things that made me grow up to be a materials scientist. Giacomo, apologies for being one of the boring people.

Oh, and d'oh! I forgot to check out the amorphous metal when I was in the lab. Sorry, Xopher - I'll try to remember to check it out later.

Oh no! That means that when Kirk, Spock, and the crew comes back to get some humpback whales, Scotty won't be able to barter a secret formula for transparent aluminum for six inch stock of plexiglass.

Egads! I never thought of that! This discovery could change the course of history! Or the future. Or something like that.

Flying car? I was hoping for a flying carpet.

Well, if you want to keep a bird in your car, that's up to you. But it's going to make driving a little difficult.

Dolloch says: NASA has a great stance on the situation which seems to be "Yeah, well..."

Well, okay. The "Bernouilli" and "Newtonian" views are complementary, like different frames of reference, and you can get to a correct answer either way. It's true that if you look at the direct force balance on the airplane, the reason it stays up is that the pressure on the upper surface of the wings is lower than the pressure on the bottom surface. And the reason it's lower is because the air is flowing faster there, and Bernouilli's law applies. But the next step in that logic -- why is the air flowing faster on the upper surface of the wing? -- is incredibly non-obvious. The Coanda effect comes in, and maybe some other stuff -- fluid dynamics has never been intuitive to me.

The "Newtonian" wings-are-ducts view is also only sort-of-correct, but it seems to me it is both easier to visualize and gets closer to the truth without going over, as it were.

IIRC the quick way to discredit the Bernoulli effect on the carefully shaped airfoil as the only thing holding a plane in the sky is to fly upside down.

Argh, the "Bernoulli is bunk" thing. *crycry*

This one drives me crazy. I'm glad that Colin Roald already started with an explanation, because I might have gone crazy if I was starting from scratch.

1. Your average jumbo jet DOES NOT produce enough thrust to keep itself aloft purely by forcing air downwards. Those are big engines, but the airplane is also very heavy. If you point a jumbo jet upright, so those big engines are working directly against gravity, the plane will not fly. (Some airplanes do have engines strong enough to do this, most obviously the Harrier Jump-Jet, which doesn't even rely on tricky vortices (and/or whatever) for lift the way helicopters do (I actually don't know anything about helicopter flight, I've just heard that "vortices" are involved, so take that for what it's worth (nothing))).
2. If your explanation begins and ends with forcing air downwards, then with the information presented in 1, you lose, you are wrong. Because there is no free lunch. Using wings to force air downward is (Newtonian) mechanically equivalent (on the body as a whole) to using jet engines to force air downward, and would require the same amounts of thrust to remain aloft.
3. Why don't those complaints stop an airplane from flying with "airfoil explanations"? Because they are using ForceA to create a pressure differential which exerts ForceB (in this case, due to the design of the wing, B > A). Which is (Newtonian) mechanically different from using ForceA to create (or cancel) an acceleration.
4. The pisser is that it's not simple. Some things in the is world are simple, airfoil flight isn't one of them. Fluid dynamics are complex (especially to somebody like me that never got down to any sort of unifying principles) and I haven't studied them [adequately]. Like Colin Roald said, you use (complicated) fluid dynamics to create a pressure differential which has creates a simple Newtonian equivalence of forces (weight & airfoil lift).

Like the "bees shouldn't be able to fly" story, the moral is that when a simple model predicts something shouldn't work, but reality demonstrates that it does, the model is leaving something out. In the case of the "Bernoulli explanation" of airfoil flight, the thing that's being left out is the creation and preservation of streamlines.

I think I am too wicked and deserving punishment for my sinful thoughts. I actually read Xopher's last comment without ever thinking about small animals with wings.

The best paper airplane in the world

I was going to point out how it can fly despite having completely flat wings, but the heck with it. Paper airplane, wahoo!

Thus far debcha: This is an awesome book that talks about how cool freshly-drawn or broken glass is; it's one of the things that made me grow up to be a materials scientist.

Seconded! (And, in fact, I opened the link thinking, that's going to be him, isn't it...) I first encountered this one when I was 17, and it made sense of a lot of my A-level chemistry and physics, and then kept right on making sense of chemistry at university. (I still remember the look on my colloids lecturer's face when I finally "got it" - and blurted out "oh, like metallurgy, I see now!")

I ended up researching nanomaterials rather than the kind of things he writes about, but it's still the same basic set of principles, and I partly blame this book for my interest in history and archaeology as well - historical manufacturing techniques and the evolution of, well, Stuff.

Cassie wrote: I thought it was manganese or magnesium, one of those M elements, that turned glass purple. Yep - manganese gives purple/pink/brown, all those shades. You see it in solutions in the lab too, and I actually think of evening city skies as being "manganese-coloured" because of it.

Most glass naturally contains iron, so classically, glass in the State of Nature is green or greenish - manganese counteracts this, and more turns it mud-coloured. Cobalt turns it blue (surprisingly enough), gold turns it rosy pink or red, and if you use uranium, you get a bright vivid green. (This information brought to you by the Glass Gallery of the Victoria & Albert Museum, one of the most amazing parts of an utterly wonderful place.)

"IRC the quick way to discredit the Bernoulli effect on the carefully shaped airfoil as the only thing holding a plane in the sky is to fly upside down."

It only discredits Bernoulli if it's the only thing holding a plane in the sky. I always thought that if you just tilted the wings up, you could fly - but it would take a tremendous amount of power - its terribly inefficient. The Bernoulli effect gets you the efficiency to make it practical.

So, if flying a plane upside down is as efficient as one flying rightside up, then I'm all wet.

Well, I had fallen for that thinner-thicker and therefore fluid talk all these years, and am actually relieved to hear it debunked.

Mostly though, I'm just beaming with delight to have learned that viscosity is measured in units called poises.
Charming word. Call me silly, but I'm as gobsmacked as my poet-father was the first time he heard me use the word "ormoulu" casually in a sentence.

Poise. What poise.
How sticky is that glue, in poises, please?
Let me measure, en pointe.
-hee!

...if you use uranium, you get a bright vivid green...

I'd hate to be a collector of 'canary glass' in this day and age. Can you imagine travelling through an airport with your latest finds - only to start some TSA radiation detector a-clicking?

"No, it's not a dirty bomb, dammit - it's my collection!"

I've seen all sorts of science books that use the Bernoulli effect. It's been persistent ever since the early days of aviation when the military, in the best sense of "military intelligence," decided to use it to explain airplane flight in their manuals because they deemed it easier to understand than the real Newtoniaon explanation. The Wrights, Otto Lilienthal, Glenn Curtiss, and just about all pioneering aviation innovators knew better, but the explanation has persisted because of the military's use and because the diagram is so pig-simple to duplicate. It's become iconic.

It's not that the Bernoullli effect doesn't occur over a wing. It's that by itself it's insufficient to explain lift.

Bernoulli diagrams nearly all show an airplane wing that is curved on the top side and flat on the bottom. I have yet to see a wing on a real airplane (or on a bird) that is shaped that way.

Even if they were, if the Bernoulli effect were all that was holding a plane up in the air, how could stunt planes fly upside-down?

If you think of a helicopter blade as a moving wing (since that's what a propeller is, by its shape and its function of moving air), and stand under it while it's moving, you can feel with no uncertainty exactly what it is that's moving that helicopter into the air.

Oooh, thank you for the book recommendation. I learned a lot about slicing and breaking during my time at the microtome, and I like things being solid.

Bernoulli's one of my favorite effects for everything because it showed up in almost every biology class I ever took. Blood cells congregate in the center of a vessel? Tunicates, sponges, et cetera with their currents? All Bernoulli. That it helps keep planes in the air is incidental.

Oh, well, if we're upping the ante to flying carpet, then what I'd really like is an oud made of crystal and dragonbone, strung with frickin' moonbeams.

(And while we're at it, an enchanted plectrum that lets me play "Discipline" on it.)

Taking advantage of my materials science soapbox-for-a-day, I'd also like to recommend Mark Eberhart's lovely book, Why Things Break, a more modern and accessible book about, well, why things break. It's only a few years old, and is a worthy update to my beloved old Gordon. Eberhart talks about everything from Pyrex to how to design plutonium-based energy sources for spacecraft so that they can survive crashes to what the difference is between Boston snow and Colorado snow.

Okay, I really have to get back to putting my mat sci test together, so I can go home while it is still tonight and not tomorrow morning.

Xopher: Where I come from, having a bird in your car would definitely distract from driving; but not for those reasons.

Dan Layman-Kennedy: That is one hell of an oud!

returning to the glass for a second, it seems entirely sensible that the 'flow' argument is both i) an intuitive explanation for the layperson observing the thick/thin phenomenon on old windows, and ii) entirely wrong. the structural argument for orienting tapered panes as base-heavy is compelling; it seems to me, though, that the link to 'proof' offered by chris clarke (in this article) only explodes the myth as far as the windows go; the article also states that glass does in fact flow. in a timeframe comparable to (if not significantly larger than) the age of the universe, to be specific. the problem here is the timescale involved, but we already knew that; the age of cathedrals is way too short to give adequate glass flow, but this does not mean glass flow doesn't happen at all. is there a 'fluidity threshold' included in the definition of a liquid? if so, chances are it is a measure relative to the lifespan of the observer.

Odd. I could've sworn I remembered an Educational Program from long ago demonstrating "liquid glass" by putting a weight on a windowpane and showing how the glass bent.

I wonder what childhood memory I'm distorting?

Consider, if you will, the obsidian layers that come out of shield volcanoes; those crack, rather than flowing, despite having had millions of years to slump into strange shapes if they so desired.

Oh, and as I recall, float glass is floated on zinc, not tin, and that the critical difference -- aside from needing about a third the distance and it costing much less -- between float and plate glass is that with float glass, it's picked up on the rollers hot, and vertically; that way it sets fast and smoothly, rather than slowly and ripply and needing to be ground flat.

Kinda like why plagiaclases have big crystals.

(I actually don't know anything about helicopter flight, I've just heard that "vortices" are involved, so take that for what it's worth (nothing))).

vortices, rubber bands, paper clips, and a particle accelerator. Oh wait, that's how I became immortal. never mine.

4. The pisser is that it's not simple. Some things in the is world are simple, airfoil flight isn't one of them.

Well, some things really are simple, like actually flying an airplane. I like the fact that birds can fly and don't bother with the explanations. I also like that with all the complications to flying, one of the most important "instruments" is a bit yarn dangling outside in the middle of the windshield for your slip indicator. Sometimes its teh little things that count.

Graydon: Thx, that explains a lot.

It's been persistent ever since the early days of aviation when the military, in the best sense of "military intelligence," decided to use it to explain airplane flight in their manuals because they deemed it easier to understand than the real Newtoniaon explanation.
You're almost approaching truth here, be careful. The truth is that SIMPLIFIED EXPLANATIONS of the lift by pressure differential are presented in text-books. The NASA articles that Doloch (Thanks Doloch!) linked above are pretty clear on the topic. The "equal transit time" explanation doesn't reflect reality. Yes, equal transit time is in textbooks. Textbooks also say that the sun is composed of very hot gasses. Neither is true, but that doesn't mean that the sun isn't very hot, or that pressure differentials aren't the primary source of lift in airfoil flight, or that the Bernoulli effect isn't important to the creation of those pressure differentials.

If you think of a helicopter blade as a moving wing (since that's what a propeller is, by its shape and its function of moving air), and stand under it while it's moving, you can feel with no uncertainty exactly what it is that's moving that helicopter into the air.
That is an utterly inappropriate analogy. Amusingly, Helicopters don't produce enough Newtonian reaction upforce (by pushing air down) to keep themselves aloft.
Check this out, and pay attention to the part about autorotation... with air moving UPWARDS relative to the blades of the helicopter, they still provide lift. You will become a source of great amusement to me if you suggest that the upflowing air supports the helicopter in the manner of a parachute or kite. (just making sure that you are adequately warned)

I remain of the firm opinion that we simply have no real idea how the universe actually works, and that the universe likes to change how it works when we're not looking (kind of personifying quantum mechanics, so to speak). We make bold guesses, which are then "proven facts" until years, decades, even centuries later, someone comes along and decides that we're wrong, and it's actually x.

I'm perfectly content to let glass be its quirky self. It's not a fluid, but it's not a normal solid either. It doesn't "flow" per se, in most situations, but I've seen fractured glass heal itself completely before (while I didn't put it under a microscope, under a jeweler's loupe at least, it was entirely gone). I don't really care what we use to explain that, I just think it's neat. :)

nmrboy:

(this conversation was too fun to stay away from)

Glass flow happens, of course, just not at room temperature. And since glass doesn't have a sharp melting point (like ice), there has to be an arbitrary 'fluidity threshold.' But it's determined by how easy it is to work the glass (blow, pour, whatever), not by our lifespans (although I guess there is an indirect relationship between our lifespans and how long we are willing to wait for our glass to spread out).

Here's a graph showing the viscosity of glass as a function of temperature (curve (d) is regular soda-lime silica glass). Note that the graph is the logarithm of the viscosity (1 = 10, 2 = 100, 3 = 1000, etc.) and that the viscosity goes up as you go up the y-axis - low values of viscosity = runnier.

The melting point is defined as 10 Pa.s ('1' on this y-axis, but the curves don't go that far down). The working point is defined as 10^3 Pa.s, a viscosity at which glass is easily deformed. The minimum temperature at which the glass can be handled but still hold its shape is the softening point, corresponding to a viscosity of about 4 x 10^6 Pa.s (or about 6.5 on the y-axis); the temperatures between the softening point and the working point are the working range. For soda-lime glass, the melting point is around 1400 deg C, and the working range is between about 700 and 1000 deg C.

Given the viscosity and the load (ie stress) caused by the weight of the glass, you can estimate the time for a given flow. Note, however, that the flow of glass at room temperature and under low loads is small enough to be not of practical importance, which is why temperature-viscosity curves don't normally go down to twenty degrees Celsius (and presumably why it took until 1998 to debunk this particular myth).

(I should credit my mat sci text for these criteria and numbers)

And then there are planes like the F-117, which is essentially a wedge with large engines and a computer that makes it fly. Without that computer, it'd be about as nimble as a brick.

Sorry, I don't know why that link didn't work. Here's the graph.

I was enchanted with the "supercooled liquid" theory of glass, because it made it seem like glass existed on a timescale all its own.

I was taking a lot of glass art classes at the time, and the idea of working in a medium that inhabited its own universe lent an added note of mystique to what was already fascinating stuff. Until an instructor debunked the pleasant fantasy by telling us the points mentioned here: old glass is uneven because of the manufacturing methods of the time, and if glass really was a supercooled liquid, then all those glass artifacts from the Ancient World should be puddles.

Now that I think about it, it's probably just as well glass isn't a mystical magical element with its own timeline. Working with glass is fraught with enough complications. Along with differences in heating and cooling temperatures, expansion and contraction rates, different vulnerabilities to heatshock, and how you can't mix glass made by different manufacturers because they all use different formulas - glass artists would probably also have to know and remember different "ooze" rates, if glass really was a supercooled liquid :)

Melissa Mead said:
Odd. I could've sworn I remembered an Educational Program from long ago demonstrating "liquid glass" by putting a weight on a windowpane and showing how the glass bent.
I wonder what childhood memory I'm distorting?

Steel bends under weight. If you piled enough heavy stuff on top of a steel I-Beam, eventually you'd have a puddle of steel below a bunch of heavy stuff (Steel is malleable (though much less malleable than iron)). This is similar to what you're seeing when glass is bent under a weight.

The question presented in the post was, will glass deform under its own weight, and will congealed (physically indistinct) middle-section droplets deform (away from nearby upwards neighbors) under their own weight. I'm not a materials scientist, but I'll take their word for it when they say "no."

Greg London said:
I like the fact that birds can fly and don't bother with the explanations.
I spend (some) days wandering around in a stupor, too impressed with how much easier it is for animals to do things than to understand how they're done. The cognitive complexity of knowing when you're looking at a car, (much less a pencil, a computer monitor, a plate of spaghetti, or a towel crumpled up on the floor) still make me want to go back into academia and study Cognitive Science. Coordination for physical action is stranger and harder than most people imagine... by contrast, knowing how long you have until you're hit by a car is simpler than you would be taught to solve that problem in trigonometry class (but it's still harder to calculate on paper than it is to know when something will hit you).

Okay, one last thing before I head home and to bed, about broken glass fusing back together.

Nabil, I wrote above about how atoms like to be bonded with other atoms. When you break a piece of glass, all the atoms on the surface are still happily bonded to the atoms below them, but they have naked bonds waving in the wind where the other surface used to be. Since glass is so brittle - that is, it barely deforms at all before it breaks - the two broken pieces can fit back together with atomic-level closeness. So if you put the pieces back together again immediately, the bonds can join up again and fuse the glass. However, if you wait any length of time, atmospheric gases will bind to the atoms (remember, those dangling bonds really don't want to be out there, and they'll bind to anything within reach) and the glass will no longer fuse.

I've never actually tried this, and now I want to go over to my lab and give it a shot...

On the subject of groovy glassy things.
In the part of Texas where I grew up, we had a lot of lightning storms with no rain. The strikes would sometimes hit in sandy patches out in the pasture, and we'd get what we called "lightning glass."
Apparently they're actually called fulgurites, and can look something like a piece of coral if dug up carefully enough.
http://www.menzelphoto.com/gallery/big/lightning6.htm
So cool.

transparent aluminate glass was created in 2004

Oh no! That means that when Kirk, Spock, and the crew comes back to get some humpback whales, Scotty won't be able to barter a secret formula for transparent aluminum for six inch stock of plexiglass. Maybe we should start a fund, have a bake sale.

No, the time line is completely consistent:

A) Kirk, Spock, Scotty et al travel back in time to the 1980s.

B) A few years later, transparent aluminum is "discovered".

Wow. I didn't mean to run away after tossing a grenade.

The bogus lift explanation that I do my best to shoot down is the one NASA labels "Equal Transit." It uses Bernoulli's principle, and only that, to explain lift. I think it's perpetuated in classrooms and textbooks because there are so many simple, dramatic classroom demonstrations of Bernoulli's principle. As others have already said at length, lift isn't that simple.

Colin Roald linked to a great explanation of lift at FIU. The NASA flight pages also have a lot of great content. Take a wander and play with their Java applets.

my favorite SF glass idea was, i think, from a Gene Wolfe book where we are told that the sand is composed of the tiny bits and pieces of what was once the glass in windows and containers. it did a great job of showing how far in the future this civilization was. I've never forgotten that description, even if I have forgotten exactly where it's from.

Most of the physics here is beyond my feeble understanding (even though I enjoy bemusedly skimming the more learned posts), so what struck me most were the few teasing comments about liquifying wood. If in Norway wood has the consistency and behavior of glaciers, this would conjure up new images for the "so I lit a fire" line in "Norwegian Wood". (Whooee, all her dern furniture melted!)

IIRC, one of the earlier computer graphics gurus (Blinn? Sutherland?) said they'd have achieved success when they could simulate a wood scarf draped over a silk table. I don't think that's exactly the kind of flowing wood that was meant, but ...

It's an important trope in Walking on Glass by Iain Banks (or is it Iain M. Banks; always get confused between the two).

The one, actually. (I'm startled that I am the first person to respond to this.) The way I heard it, he was told at one point by a Publishing Person that he needed a different name for his SF stuff than his non-SF; the middle initial was his solution. Amazon tells me that the M. is in SF books; I will probably reforget this information immediately.

Scott writes: Amusingly, Helicopters don't produce enough Newtonian reaction upforce (by pushing air down) to keep themselves aloft.

Do you have a reference for this, or for how autorotation actually works? The idea that rotors would continue turning in the same direction after the flow of air reverses seems entirely counter to my experience with fans.

A) Kirk, Spock, Scotty et al travel back in time to the 1980s.

B) A few years later, transparent aluminum is "discovered".

Why would anyone who is sitting on a discovery worth billions of dollars stick it in a drawer for a couple of years? I have operated under the assumption that as long as transparent aluminum is not available, that means Kirk and crew haven't come back in time to get the whales yet. Once it becomes available and is "discovered", then I'll know that Kirk was just here (or in time-travel speak, Kirk was just now.)

Why would anyone who is sitting on a discovery worth billions of dollars stick it in a drawer for a couple of years?

Not sticking in a drawer - going through the patent process. And probably developing the large-scale manufacturing processes, too. At which point the Monterey aquarium replaces the current version of Scotty's windowpane with a sheet of transparent aluminum and we find out how good it really is.

Do you have a reference for this, or for how autorotation actually works? The idea that rotors would continue turning in the same direction after the flow of air reverses seems entirely counter to my experience with fans.

autogyros are aircraft with propellors on the front and a helicopter rotor on top. The rotor is only powered while the aircraft is sitting on the ground, and that is simply to get the thing spinning. Once spinning, the clutch is released, and the rotor remains unpowered for the rest of teh flight. This can be seen to be intuitively true because if the rotor were powered, there would have to be an anti-torque tailrotor to keep an autogyro from spinning while in flight.

The main rotor gets energy to spin by the airflow that goes up through the rotor, causing it to windmill. This also creates lift, which keeps you from converting into a small grease spot on the ground.

The propeller on the front of an autogyro allows you to add energy that is lost by drag, inefficiencies, and what not. It also allows you to add extra energy so that when you climb, you can maintain the same airflow up through the rotor system and keep the amount of lift.

When a helicopter loses its engine, you pull back on the cyclic like reining in a horse and drop the collective. Pulling back forces the air to go up through the rotors and dropping collective reduces drag so you don't lose rotor RPM. In a second, you've gone from powered flight with air going down through the rotors to unpowered flight with the air going up through the rotors. How the hell this works from an aerodynamic point of view, I have no clue. I am like the bird who can fly but doesn't know the math.

In a helicopter, you then trade off altitude-energy to offset your rotor-drag energy that you're losing. This allows you to autorotate unpowered safely to the ground from high altitudes. As you autorotate, you must maintain forward velocity to keep the airflow going the right way. up and through the rotors, which keeps them spinning.

If you lose an engine while you are hovering, I seem to recall that the procedure is to go cyclic forward because you have to achieve forward velocity or you're dead, and drop collective as before. I'm a little rusty on that because it's just stupid to hover at altitude.

Then when it comes to actually landing, you are basically heading towards the ground, nose down, gliding in, and around 30 feet from the ground, you pull back on the cyclic which causes the helicopter to point up, the air is suddenly pushing on the blades like a windmill, which spins them a bit faster and stops your forward velocity. The faster spinning will actually cause you to gain a bit of altitude, and when you've bled off the last of your forward velocity, you cycle forward, and land straight down, cranking the cyclic just before you crash, er, touchdown.

Take a spoon, put the scoop end on the table, and hold the handle end in the air at a 30 or 40 degree angle. turn it so the scoop end is upside down. And that approximates your ideal flight path when you lose an engine in a helicopter.

Not sticking in a drawer - going through the patent process.

Hm, that could be. OK, so as long as transparent alumninum is not available on teh market, I can assume that Kirk and company didn't come back to get the whales over two years ago or more. He may have come back in the last two years. Or, he may not yet have come back.

The important thing, though, is that the Star Trek universe remain true.

when you've bled off the last of your forward velocity, you cycle forward, and land straight down, cranking the cyclic just before you crash, er, touchdown.

ack! that should say "cranking the collective" not cyclic. Good grief, I hope no one had an engine out after reading that. I'm a bit rusty.

Greg London writes: When a helicopter loses its engine, you pull back on the cyclic like reining in a horse and drop the collective.

"Dropping the collective" means changing the angle of attack of the rotor blades?

Meanwhile, I remain unconvinced that this from Scott can possibly be true: "Amusingly, Helicopters don't produce enough Newtonian reaction upforce (by pushing air down) to keep themselves aloft. Check this out, and pay attention to the part about autorotation... with air moving UPWARDS relative to the blades of the helicopter, they still provide lift." Greg's comment that "you have to achieve forward velocity or you're dead" makes me question whether the airflow is actually upwards relative to the moving rotor blades, but even if the air has relative upward velocity before encountering the blade, it may still be redirected to a downward velocity in the course of the encounter. Fundamentally, if you draw a surface of integration at some reasonable distance around the chopper, *something* must balance the tonne of excess mass contained within that surface. Either somehow the chopper maintains a substantial pressure gradient that can integrate out to counter a tonne of force over an arbitrary enclosing surface, which I think is impossible, or else (1 tonne)x(10 m/s/s) of air is somehow being accelerated downward through the surface. Fluids are weird, but you still don't get a pass on Newton's laws.

"Dropping the collective" means changing the angle of attack of the rotor blades?

Yep. both cyclic and collective change angle of attack.

The cyclic changes it so there is more angle on one side than another, causing the helicopter to tilt away from the big angle (more lift) side.

The collective changes the angle on all blades simultaneously, so the helicopter goes up or down.

Rotor RPM is constant, so to go up/down you change the collective pitch of the blades. and to go forward/backward/sideways, you move the cyclic.

makes me question whether the airflow is actually upwards relative to the moving rotor blades,

Hm, there's a diagram here.

When you're flying, you generally don't think in terms of integrals and gradients. You're thinking "airspeed indicator, altitude indicator, rotor RPM, engine rpm. Big sky. repeat." Meanwhile, your hands are constantly trying to balance a thousand pound spinning plate on a yardstick.

I don't know the math of autorotation, but the thinking involved in actually performing one is the function that graphs the line from "oh f**k!" at the origin and "shiiiiiiiiiiiiit" at the end point.

ah, bigger drawing here.

note the coordinate axis is labeled "plane of rotation" and "axis of rotation". The x axis is the rotor disc, the y axis is the shaft that turns it. The arrow marked "relative wind" goes from lower left to upper right. It goes up through the rotor disc.

also, the little diagram on the right shows the aircraft, the rotor disc tilted back, and the airflow going up through it.

My brother =taught= autorotation when he was a helicopter flight instructor for the Army -- so imagine not only deliberately turning off your engine to practice it, but being the "driving teacher" sitting in the copilot's seat while some greenhorn pilot tries it for the first time....

(Note to self, remember to admire brother the next time I see him.)

Early industrial-era glass was made by floating molten glass on a bed of molten tin.

later someone mentions zinc, not tin.

No one ever bothers to explain why the heck the tin is molten or why you wouldn't pour the glass on something that isn't molten but really smooth.

Ah, so the incident angle of the wind is shallowly up through the disc. But I don't think that figure is drawing streamlines of the airflow, just the angle of incidence. I would wager the flow is bent substantially downward after contact with the blades.

And I'll speculate that some game played with the cyclic is what lets you use a mostly-forward-incident-wind to keep the rotor spinning even when unpowered. Would that be the effect of "pulling back on the cyclic," which Greg says is the first thing you do when thrown into autorotation? I think it might be.

Colin Roald:
Either somehow the chopper maintains a substantial pressure gradient that can integrate out to counter a tonne of force over an arbitrary enclosing surface, which I think is impossible, or else (1 tonne)x(10 m/s/s) of air is somehow being accelerated downward through the surface. Fluids are weird, but you still don't get a pass on Newton's laws.
I wish I knew enough about helicopter flight to answer your question. And I'm very glad that Greg London stepped in with answers I don't have even vague access to.
The autogyro article that Greg London linked shows the rotor blades as airfoils, so that maybe helps? I really don't know though, sorry. I'm just a big pile of second, third, and aribtrarily half-remembered information about flight.

I'll speculate that some game played with the cyclic is what lets you use a mostly-forward-incident-wind to keep the rotor spinning even when unpowered. Would that be the effect of "pulling back on the cyclic,"

Trees pretty. Fire bad.

(someone on this board should recognize that reference and to what end it was made.)

Uhm, I think the deal is that when you're moving under power in a helicopter, you're nose down and the wind is going down through the rotor disc. When you lose power, you need to get the air going up through the disc and you pull back on the cyclic to rock the fuselage back.

A quick google of both tin and zinc gives tin with a melting point of 231.93°C and zinc at 419.53°C. Cooler than molten glass, at least. (I want to say 'way cooler' but it doesn't seem to fit. Glass is way cool, though. I've seen some Roman stuff; having been buried, it hadn't changed color much.

I am perfectly willing to believe glass flows down despite the facts. I realize this belief is a slippery slope that ends in enormous oil profits, an increased federal deficit and lots of dead soldiers, but, at least in this case, our children will think it was cute.

Oh, and planes don't really fly. Big metal things in the air? That's just crazy. It's magic.

No one ever bothers to explain why the heck the tin is molten or why you wouldn't pour the glass on something that isn't molten but really smooth.

I'm speculating, but could it be because nothing that isn't molten is as smooth as molten metal?

Greg:

As far as I know, it's normally molten tin, not zinc, that's used to make float glass (sources are the mat sci textbooks I've mentioned above, as well as Wikipedia).

Early sheet glass was, in fact, made by pouring glass on very flat sheets of metal (like iron). And the reason why you use molten metal is because a layer of liquid on top of another liquid allows you make continuous sheets of unbelievably smooth, uniform glass (as evidenced by the size and optical clarity of the glass in the average curtain wall).

can't... wrap... mind... around... molten... metal... and... glass...

SNAP!

ow.

Nope, my brain refuses to accept it. I reject your reality and substitute my own.

Is that fanfic? Or religion? LOL

Can anyone discuss the use of zinc vs tin for float glass? IIRC, zinc is useful in tiny doses but toxic in larger ones (the late Johan once had to spend some time drinking extra milk to counter the effect of a crucible of zinc ~boiling over); somehow the phrase "mad as a plate-glass pourer" never made it into the language.

If wood isn't liquid, then what is in those cans and tubes labelled Liquid Wood?

J. Austin: Fulgurite is indeed pretty cool stuff. There's also trinitite, which is sand fused to glass by having your friendly neighborhood nuclear device go off next to it.

Jim K: The beach sand that is glass pounded down by the clangorous sea is from Book of the New Sun, not too surprisingly.

Re Bernoulli, flight, and science books: long ago, when Philip and Phylis Morrison were still reviewing books for Scientific American, they did a broad survey of science books for kids. They were less than amused by much of the material.

And as for the time lag between the arrival of the Enterprise crew in SF* and the public announcement of transparent alumin(i)um, a more stefnal explanation was covered by John Campbell decades ago in a well-known editorial, "No Copying Allowed;" it ain't easy to reverse-engineer a manufacturing process from a technology significantly more advanced than your own, even if you have a sample to hand, and in some cases, even if you have a description of the manufacturing process.

"How did the strangers with the weird personal habits make such metal? With weapons of that mighty steel we could conquer Hounslow, Putney . . . perhaps the Tootings of Bec shall fall beneath our chariots."
"Once we get the wheel thing down."
"Eh?"
"Nothing. Their jester said something about 'basic oxygen furnaces,' o Wonder of Nature."
"Then build furnaces, lout of a knavish churl! And gather as much of this 'oxygen' as you can. Perhaps it grows in peat bogs."
"You are not called Dymwyt the Wise for nothing, Your Liegeyness."

*the city, dammit.

it ain't easy to reverse-engineer a manufacturing process from a technology significantly more advanced than your own

Yeah, but Scotty had the chemical formula right there on the screen. I could see the molecular drawing floating on the monitor. If it wasn't too advanced for the software to simulate it, then it can't be too advanced to build the real deal. If it was out of our technology range, then our software would have told Scotty, "invalid operation, stack dump follows: oop". Instead, it drew it, and even labeled it "Transparent Aluminium" right there on the monitor. Therefore, I'd say there would be, at most, a 2 year lag for patent delays between Kirk-saves-whales and the appearance of transparent aluminium on the free market. It's the only valid explanation.

Making the molecules for transparent alumin[i]um may be more difficult than getting the program to draw them - bond angles can be looked up - but I would like to know how they got it labelled!

Having worked on some old houses, I can testify that shingle wood does flow -- upwards. Shake wood, on the other hand, does not perceptively transform after it is froed from the billet. Flooring wood, however, does not always flow to absolute smoothness within a reasonable time-span if it is subjected to heavy wear. All of this is, of course, independent of the fact that all woods (AFAIK) expand and contract (as the result of changes in moisture content) far more across the grain than with it.

Evidently some types of cookware in India were traditionally made of tin but have not survived the transition to modern stoves, which tend to kill them; this article mentions something called an "iya pathiram" for making a particular sort of tamarind/tomato-based soup, though I'm somewhat disturbed by the comment that without the tin vessel, "Rasam has never tasted the same again." But I've never managed to melt glass on a stove, though possibly I just haven't tried hard enough.

(Found the link while trying to identify the original culture of a modern replica cookware set made of brass/bronze; no luck yet-- a wide lipped platter, a kettle with a very narrow bottom, and two long-handled pans, one nearly flat and other other round and shallow. Do these ring any obvious bells for anyone?)

I can't imagine trying to get aluminum to be transparent, even with the molecular formula. Chemists seem to spend a lot of time working on getting a sulfur here or there, or making a molecule with a certain chirality, or trying to find the right enzyme to do it all for them. And then it's all about yield.

About the musical properties of metallic glasses: This should only be relevant for percussion instruments, and not for instruments where the metal is used as a reverberating cavity (i.e., woodwinds and guitars, lutes, etc.).

IIRC, there was an experiment some years ago which conclusively showed that the different sound properties of instruments made of different materials were only due to differences in shape coming from the manufacturing process. To prove this, the experimenter made concrete flutes which were very precisely machined so as to sound like a wooden flute.
(Googled it, the researcher was John Coltman)

The man bent over his guitar,
A broad-hatted dude. The day koo-kachooed.

They said, "You have a brick guitar,
You do not play things as they are."

The man replied, "Things as they are
Play slide upon the brick guitar."

And they said then, "But play, you must,
A tune beyond us, as we am,

A tune upon the brick guitar
Of things as-is, with DRM."

"I cannot roll the world, though round,
Although I oil it as I can.

"I sing a hero's funneled head,
A mighty axe, no heart contained,

"Although I oil him with this can
He still gets rusty in the rain.

"If strumming bricks should trouble you as stark,
Let me tell of the swans, that they live in the park;

"Say it is the serenade
Of one whose instrument got laid."

-- with all the usual apologies.

Oh, lotsa good stuff, and no time to comment (I gotta read ML more often...). I'll just note that a helicopter in autorotation may be easier to figure out if you think about gliders (sailplanes) which, in the absence of updrafts, do the same trick of slowly using up their potential energy (altitude) to maintain their forward velocity. You can model a helicopter in autorotation as two or three gliders flying in small circles, with their wingtips tied together (now *there's* a pilot's nightmare). As far as helicopter lift being "non-Newtonian"... bah, humbug. Momentum is conserved; out of ground effect, the only way you get lift is by accelerating air downward. Wings are better than jets standing on their tails because they interact with a larger volume of air: large volume => large mass => small velocity change for a given force => small amount of energy needed per unit momentum (since energy is proportional to velocity^2).

There's a way-cool exhibit at the Hiller Aviation museum on the SF (the city) peninsula about a proposal, during the early days of the Apollo program, to recover Saturn V booster stages by parachute and reuse them. To avoid dunking the stages in the ocean, they would be plucked out of the air by a *giant* helicopter, with 100-yard-long blades (IIRC) spun by pairs of large jet engines at the tips; the blades spun at something like 10 RPM. They have a video made from the "demo" film, which, in those pre-CGI days, used wooden models hung on wires...

Oh, and metallic glasses? Also known as "microcrystalline" metals. Made by rapid solidification of metals; one way is to shoot a jet of metal against the rim of a rapidly-spinning disk. One of many jokes about the National AeroSpace Plane project (aka the Full Employment for Computational Fluid Dynamicists program) is that, while lots of aerospace designs require Unobtanium to work, the NASP needed Rapidly Solidified Microcrystalline Unobtanium.

It's been brought to my attention (by the part of my mind that was especially active at 4 a.m.) that my previous statement "..shingle wood does flow -- upward" was misleading. Shingles are, typcially, heavier at the bottom; it's just that the older ones I've observed have all been thicker, throughout, than newer ones.. And shake wood apparently does sometimes flow, depending on how the shakes were rived {or riven, froed, frowed, or split -- this is an ancient Art & Craft, with various regional usages). Small splinters of shingle wood, of course, are well-known to tly -- usually into people's eyes or down the collars of shirts.

Two comments, somewhat belated and not entirely on topic:

1) At Wiscon 14, Iain Banks explained the 'M' as being a marketing decision by his publisher. At a later con, his publisher denied that and said the M (for Menzies) was inserted for the SF novels to placate his family (Banks is not his family name). I still tend to think it was a Stupid Publisher trick, but then I work in the industry. ;-)

2) My Google-foo is failing me, but I distinctly remember an incident in Milwaukee where a heavy lifting helicoptor was transporting an air-conditioning unit for a new building and lost the rear rotor. The pilot (Viet Nam vet) was able to not only use the remaining controls to guide the cargo to a nearby sandpile and drop it without significant damage, but then guide the helicoptor itself to a softish landing (copter destroyed but pilot and all bystanders unharmed).

John

John M Ford: The sound you heard was Wallace Stevens laughing delightedly beyond the grave.

Jordin Kare writes: They have a video made from the "demo" film, which, in those pre-CGI days, used wooden models hung on wires...

Oh my god. I must see. You don't have a link, do you?

concrete flutes which were very precisely machined so as to sound like a wooden flute.

OK, will people please stop posting stuff that hurts my brain? I'm still a little dizzy trying to imagine a window pane brownie in the oven with a molten tin crust and a filler made of two thousand degree glass and it all coming out perfectly smooth.

So, I swear I just saw a program on TV about making violins and these guys were shaving the wood to certain thicknesses because the top wood panel in a violin is built to vibrate and amplify the sound. Now I'm trying to picture a concrete violin and it's hurting my brain.

Greg --

It wouldn't work; violins are strings, not woodwinds.

With woodwinds, it really is the vibrating column of air, rather than the vibrating instrument, which is why plastic clarinets and bagpipes -- Delrin is wonderful stuff -- sound every bit as good as the wooden ones. (Also cost less and break less....) This is also why the concrete flute would work fine.

A concrete violin, or any other stringed instrument where the body of the instrument has to vibrate to produce the tone, wouldn't work well at all.

Graydon: I'm a clarinet player. Plastic ones don't sound as good as wooden ones.

Someone ran a comparison study of the acoustics of flutes made of various metals running all the way up to solid platinum. I'd've liked to see the funding application for that one. Eeeee.

TexAnne --

All the blind tests done with bagpipes in the last decade or so would indicate that people really can't tell.

I don't know if this is true of clarinets, but parallel experience would suggest that it really matters if the listener knows what the instrument is made of.

There might not be any high-end plastic clarinets being made, either, which -- in terms of manufacturing precision and care -- really matters.

A concrete violin, or any other stringed instrument where the body of the instrument has to vibrate to produce the tone, wouldn't work

er, OK, then what instruments have the body vibrate to produce the tone? Is it just stringed instruments? Everything else makes sound by a vibrating column of air? I could have a concrete trumpet? I suddenly have the odd sensation of knowing just how little I know about how musical instruments work.

OK, then what instruments have the body vibrate to produce the tone?

Drums & other percussion instruments? (Guessing here...)

Drums, yes. Any stringed instrument (including pianos and their cousins).

The instruments that involve a column of air would be woodwinds and horns...and do calliopes count even though it would be steam not air?

Are pipe organs considered 'wind' instruments? I've forgotten what the mechanics are that make the sound for these...

This should only be relevant for percussion instruments, and not for instruments where the metal is used as a reverberating cavity (i.e., woodwinds and guitars, lutes, etc.).

Well, first: you mean the percussion subclass of ideophones, since membranophones (drums) use the body as a reverb cavity.

Also...I'm dubious that guitars and lutes are in that category. I don't think a guitar made of rosewood sounds the same as a guitar made of pine (or plastic). Different materials can have different resonant properties even as a chamber. I want to see his data. Also, what's true for a high-pitched instrument like a flute may be less true, or not true at all, for a deep-toned one like a contrabassoon, or even a guitar.

Julie L.: bizarre as it may seem, you can borrow a platinum flute from someone who already has one. There's a reason Varèse titled his piece Density 21.5.

protected static, drums have a membrane that vibrates, but also a column of air in a resonant chamber (depending on the drum). Anyone who thinks the air column doesn't contribute significantly to the sound has never seen anyone playing a doumbek.

A concrete violin, or any other stringed instrument where the body of the instrument has to vibrate to produce the tone, wouldn't work

A carbon-fiber stringed instrument, on the other hand, works just fine, and has the benefit of looking like Darth Vader's violin. (Apparently they don't sound as good as the very best wooden instruments, but they sound much better than wooden instruments in the same price range).

I don't think a guitar made of rosewood sounds the same as a guitar made of pine (or plastic).

Hm, so I recall a compony that built electric guitars out of metal (aluminum, I think). A guitar buddy of mine thought that the electric pickup in a wood guitar might move with the wood, but that it wouldn't move in metal, and thought it might sound different. Alternates were that the vibration of the string would go down the metal body a lot more and could drastically alter the way the strings vibrated, would vibrate the pickup, and ultimately, teh way the guitar sounded.

We could never test the theory, because the metal guitars were too expensive for college students.

I liked the idea of a metal guitar just because the thing would be well near indestructable, and wouldn't shift adn warp as you went from 10 degree below and dry air in you car in the winter, to 70 and humid in somebody's apartment.

carbon fiber cello, $5,000
being able to throw it to the ground in frustration and know it won't break, priceless.

drums have a membrane that vibrates, but also a column of air in a resonant chamber (depending on the drum)

I guess I'm having a hard time seeing how to separate out the 2. If the body of the drum contains the column of air, how does that differ from the body vibrating? And wouldn't the material from which the body was constructed potentially change that vibration?

(Not questioning you - just thinking out loud (thinking in pixels?).)

no carbon fiber guitar, though. Perhaps the rockers who could actually afford it would want something a little more fragile so it breaks nicely when smashing it against the stage.

protected static: blow across the top of a bottle. That's a vibrating column of air inside the bottle. Clink the bottle with a fork. That's the body of the bottle vibrating.

Ever look at a vibraphone? Both principles are active there. The ideophonic part is the actual "key" of the vibe, the part struck. Beneath it is a sounding chamber, partially closed by a rotating disk. As the disks spin on an axis, this changes the length of the column of air, causing the characteristic vibrato sound of the vibe.

The chambers themselves only contain the column of air. The "keys" are vibrating themselves, and transferring part of that vibration to the column of air.

Greg: actually, carbon fibre reinforced plastic is stiff and strong for its weight, not in an absolute sense - I think your carbon fibre cello would smash as dramatically and irrevocably as a wooden cello if you threw it to the ground with any force. I have a friend who was in a bad rowing accident and took a carbon fibre oar to her skull. Her skull survived intact (although she had issues for about a year afterwards). The shattered remnants of the oar hang as a trophy in her apartment.

Colin Roald, re the simulation of helicopter recovery of a Saturn V booster:

Oh my god. I must see. You don't have a link, do you?

Sadly, it's not online to my knowledge. You'd have to go to the
Hiller Aviation Museum to see it -- the museum is definitely worth a stop if you're ever in the area. They have lots of other exotic helicopters and aircraft, some one-of-a-kind.

There's a mention of the Saturn recovery plan online here
with slightly different numbers than I recalled, but no pictures, alas.

The liquid-wood theory also explains why trees don't go higher than the timber line.

Wood is liquid because it has phloem.

The Hiller museum was just around the corner from my old job. In fact, it was the site of the last division party before we were all kicked out of Oracle. Ah, happy memories.

The Saturn V booster recovery proposal struck me as a desperate "how can we use helicopters in the space race" notion.

Definitely worth a trip if you're going up 101.

Metal guitars are made by the National Guitar Company.

See also Guitars with Metal Resonators: Dobro.

debcha: I think your carbon fibre cello would smash

Well that's dissappointing, I wonder if its too late to cancel the order...

The various articles referenced behind that link include a fellow who wanted a carbon fiber cello to enable him to provide incidental music while white water rafting; the adjective used is "indestructible".

I think it's good odds that in order to have enough mass to be playable, the carbon fibre instruments have to be much, much stronger than the minimum structural strength requirements to string one.

Just think, in a decade or so, they'll probably be able to do carbon nanotube musical instruments.

re: saturn rocket booster recovery

I recall seeing a proposal for "behind enemy lines" recovery for downed airmen and special ops. The grunt would inflate a weather balloon with a couple hundred (thousand?) feet of line attached to it. This would clear any trees and what not. Then, a C-130 would come along, low and slow, with a big, v-shaped cable catcher on it's nose. The initial tug on the guy on the ground would be straight up (in the proposal, at least), so it would pull the guy straight up and out of the trees. The initial tug would also be small and get progressively larger as the aircraft went further along. Then once the guy is fully off the ground and moving at the same speed as the aircraft, they'd winch in the cable and bring the guy in through the back loading ramp.

Apparently, this approach would solve the "aircraft exposed to enemy fire while hovering to pick up the grunt" problem.

I don't know if it was scrapped because the initial force was actually bigger than expected and they'd end up winching in a torso with no arms or legs, or if somebody realized that being behind enemy lines with a big ass balloon hovering over you telling everyone "helpless downed airman here" was a bad idea.

But it was good fiction.

dave: Metal guitars are made by the National Guitar Company.

The guitar I rememeber seeing was made entirely out of aluminum, including the neck. THat is generally the bit that warps and shifts with changes in temp and humidity. The guitar had no body to speak of, it was just a long, skinny, bar-shaped thingy, made entirely out of aluminum.

I did some googling and can't seem to find anything that looks it. I do recall pondering whether being able to control the warp of the neck through the truss rod (a bolt going through the neck) is something that could not be replicated on an all aluminum neck. Maybe it turns out aluminum doesn't work well for allowing minor adjustment. I know some guitarists are very sensitive to the way their rig is set.

I have a les paul electric, a yamaha acoustic, and an electric travel guitar (I can't remember the brand), so I don't really need another one. I might have replaced my travel guitar with an all alumninum electric, but oh well.

Re carbon fiber string instruments: what would happen if a neo Jimi Hendrix decided to light such a guitar on fire? I don't have the physics to figure it out.

Greg: Not fictional, just less and less practical:

Col. Allison Brooks, then Commander of the ARRS, and A3C Ronald Doll participated in the first human testing of the Fulton surface-to-air two-man recovery kit at Edwards AFB, California in May 1966. Recovery kits were designed for one and two-man recoveries, but eventually proved impractical for most rescue purposes. By 1996 the 8th SOS was the only unit in the world that maintained crew proficiency in the use of the Fulton recovery system, and had been prepared to launch if called upon since the late 1960's. A fatal accident in 1982, the only fatality in 17 years of live pick-ups, damaged the credibility of the personnel pick-up system within the special operations community. That, along with the increased availability of long-range air-refuelable MH-53J Pave Low and MH-47E Chinook helicopters, and tightening budgets, caused AFSOC to deactivate the capability in September 1996.

Evidently, the Fulton was an evolution of an equipment-recovery system the US developed during WW2... Come to think of it, I recently saw something on the History Channel or the Military Channel showing C-47s swooping down at insanely low altitudes with hooks to snatch the tow lines of gliders and yank them airborne again. Evidently this was used for medevac (!!!).

The Mississippi delta
Was shining like a nanomech guitar
I was following the schema
Through the cradle of a metaphor

This could go on and on.

Greg London:

Ovation guitars, while not made of carbon fiber, are made of a composite synthetic material, and have been around since the sixties.

Oh yes, I thought about getting an Ovatin a couple years back. It just didn't do it for me. Don't know why. I figured it was fancy fiberglass. That didn't bother me, I just wasn't into them.

The balloon recovery thing? You can see it demonstrated at the end of Thunderball, IIRC, as James Bond is picked up and reeled-in to a transport plane of some kind. (Not a C-130, I think, but the memory is fuzzy.)

Mike: You missed out one apology - I think your brick guitar poem just brought back my headache. Ow.

To quote an old alt.folklore.urban tagline, Eppur si fluove.

Sound properties of metalic glass: google "Glass Harmonica".

Part of the colour differences in window glass USED to be because they found that a small amount of lead added to the mix gave a clearer glass (still used in ornamental "lead crystal" cut glass. The lead gave the purple tint, rather than the green tint of flint glass.

Typography is interesting, but it's still spam