It seems that both Notso Fantastic and Genghis may both be right. Following a bit of googling, it seems that the majority of glass for windows produced up until the mid nineteenth century was made using the
Crown Glass technique. Basically a big bubble of glass flattened out. This led to the glass being thinner nearer the edge. The common practice at the time was to put the thick side down.
So Genghis take a bow for the "thicker at the bottom theory"
and Notso Fantastic for the "Glass isn't liquid" statement
other URL's
The London Crown Glass Company
A Usenet Physics FAQ