I always thought with really big panes it appeared to vary from time to time, sometimes in and sometimes out, much like belly buttons, and had to do with the relative air pressure differential between indoors with giant mechanical atmospheric controlled systems and outdoors with a God controlled system.   The other reason that struck for constant concavity was the old style vacuum sealed thermopanes with their constant inner negative pressure against the higher pressure of the outer world.

cp in airless painless bc
  ----- Original Message ----- 
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  Sent: Saturday, February 16, 2008 1:11 PM
  Subject: [BP] The larger modern windows ...


  ...one of their negative features is the shallow concavity they exhibit, which alters the reflections you see from the street.  (A separate issue is the "fuzzy" kind of reflections you get from the double-glazing.)  

  Why does the glass have to bow in like that?   ---christopher





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