bl@hplabsb.UUCP (Bruce T. Lowerre) (11/05/86)
In article <703@chinet.UUCP>, magik@chinet.UUCP (Ben Liberman) writes: > > I have also heard that the large store front windows on the downtown Chicago > stores (Marshall Fields, etc.) would be removed and rotated every few decades > to prevent breakage because the glass would get thicker at the bottom and > thinner at the top. There has been some comments on the net about glass windows flowing over long periods of time. If glass really does flow over time then what about old telescope mirrors and lenses. The Mt. Wilson observatory has a telescope with a 100 inch mirror (made of plate glass) which is about a 100 years old. Also, the Yerkes (SP?) observatory has the largest refractor telescope with a lens of 40 inches including a double element, one made of flint glass and the other made of crown glass. One element has a thick center with thin edge. I have not heard of either of these glass optical instruments suffering distortion due to glass flow. Even the SLIGHEST glass flow (1/4 wave length of light) would distort the objective element beyond use. Does glass really flow at room temperatures?
dmu@lcuxlm.UUCP (Murphy Douglas) (11/18/86)
About this matter of glass flow---yes it does occur. Back in
the good old days when I was a student at Case Western Reserve,
we had our Physics lectures in the old (1896, I think)
Rockefeller Building. In the top floor lecture room, there
were some original windows remaining that had panes ~4 feet
high. These windows had a remarkable characteristic: They
were more than twice as wide a short distance (1/2 inch or so)
from the bottom than they were near the top. Yes, glass is a
liquid, and does flow---but it has one of the highest
coefficients of surface tension known to man, even to the
point of giving it rigidity that will last for years.
---Doug--- {ihnp4 | akgua}!lcuxlm!dmudgary@ecsvax.UUCP (D Gary Grady) (11/19/86)
I have seen frequent reference to windows being thicker at the base than
at the top as an example of glass flow. However, I have also heard that
glaziers make (or used to make) large panes of glass thicker at the
bottom for some reason, so that those panes looked that way when they
were originally installed. One test of that theory would be to look at
the frames. Anyone have solid information on that?
--
D Gary Grady
(919) 286-4296
USENET: {seismo,decvax,ihnp4,akgua,etc.}!mcnc!ecsvax!dgary
BITNET: dgary@ecsvax.bitnetlew@ihlpa.UUCP (Lew Mammel, Jr.) (11/22/86)
A big problem with the thicker-at-the-bottom concept is that the top of the window is still at its original height. If the glass pane is modeled as a viscous flow field, you're going to get tremendous sagging of the top edge. This is not to mention buckling of the plane surfaces. I don't think you can model a free standing glass slab with Euler's equation for viscous flow! Lew Mammel, Jr.
dmu@lcuxlm.UUCP (Murphy Douglas) (11/26/86)
Referring to previous articles on glass flow, I mentioned in my earlier posting an example from personal experience a few years ago. This window was NOT thickest at the very bottom, but a short distance (1" or so) above the frame, which was made for a uniform thickness window. Doug Murphy AT&T-Bell Laboratories (lcuxlm!dmu)
usenet@ucbvax.BERKELEY.EDU (USENET News Administration) (12/03/86)
the relevant factor in whether glass is a liquid or solid isn't whether it flows or has crystalline structure or obeys euler's equations. mechanical engineers get around the problem by calling it a "viscoelastic" solid. it certainly is possible to have a noncrystalline solid and to have a fluid which fails to satisfies euler's equations (which apply only to newtonian fluids). From: dma@euler.Berkeley.EDU (Controls Wizard) Path: euler.Berkeley.EDU!dma