Just a note: Cam mentioned that glass was a very slow flowing liquid. This is a very common misconception. This is a good link explaining that, though the arguments are kind of dense.
http://www.weburbia.com/physics/glass.html
If you want to ignore a lecture on thermo, you can just trust me. I'm a chemical / materials engineer. (B.S. from Rensselaer Polytechnic Institute in Chemical and Biological Engineering, M.S. from Worcester Polytechnic Institute in Materials and Process Engineering.)
Also, the reason for very old glass being thicker on the bottom (which is explained in the link provided) is that they used to use techniques that didn't give very uniform thickness. So, they installed the thicker (heavier) end down because it just kind of makes sense to put the thick bit at the bottom. Gravity and all. This is highly fragile glass we're talking about.....
And if you don't want to trust me, google it a bunch. Many sci/tech websites out there for you to find with explanations on the issue.
Sorry to call you out Cam! Hope it didn't come off dickish!