The Answer:

For years people have looked at 12th century medieval glass windows and determined that the reason some are thick at the bottom is that over time gravity causes the glass to "flow" towards the bottom of the frame.

Although glass may "flow" if heated enough, this idea that there would be recognizable flow at room temperature after a few hundered years is completely untrue.

Physicists who have put this theory to the test say it would take millions (not hundreds or thousands) of years for there to be any noticeable change in the glass at room temperature. A study published in the American Journal of Physics went so far as to say that the period this phenomenon would require is "well beyond the age of the universe."

The variations in thickness in these windows can, instead, be attributed to how they were manufactured. Apparently it wasn't easy making flat plates of glass centuries ago.

Sometimes glass would be poured into molds. Other times glass blowers would make a sphere and then spin it vertically very quickly to flatten it out. Niether technique resulted in perfectly flat panes of glass. Either way it would make sense to us that a window maker would install the thicker end of the glass at the bottom of the frame.

Here is a link to more information about the science of glass: Physics FAQ

—The Editors