why is glass of window panes of very old buildings found tobe thicker at the bottam than at the top?????? 

Glasses in the windows of old buildings are found to be thicker at the bottom because of the manufacturing process of glass used at that time. 

Earlier process of manufacturing glass involved spinning molten glass into a hollow globe. This was later flattened by reheating and centrifugal force. So, the glass so manufactured was thinner at the outer edge but became progressively thicker as we moved to the center i.e. the glass produced was not complete flat.

This glass was then cut into desired size and shape. Thicker edge of the glass was kept at the bottom of windows attain more stability of the windows.