The glass in an old window will be thicker at the bottom than at the top due to the glasss viscosity. Granted, the difference is slight and it doesnt manifest until many years have passed, but glass is a liquid, albeit a highly viscous one.
And the reference for this is?
53 posted on 04/20/2016 8:37:44 AM PDT by imardmd1
(Fiat Lux)