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any case very obscure.
Lines 19-23 are on the right hand margin close to the two sketches
on Pl. CII, No. 3.]
7
72.
OF CRACKS IN WALLS, WHICH ARE WIDE AT THE BOTTOM AND NARROW AT
THE
TOP AND OF THEIR CAUSES.
That wall which does not dry uniformly in an equal time, always
cracks.
A wall though of equal thickness will not dry with equal quickness
if it is not everywhere in contact with the same medium. Thus, if
one side of a wall were in contact with a damp slope and the other
were in contact with the air, then this latter side would remain of
the same size as before; that side which dries in the air will
shrink or diminish and the side which is kept damp will not dry. And
the dry portion will break away readily from the damp portion
because the damp part not shrinking in the same proportion does not
cohere and follow the movement of the part which dries continuously.
OF ARCHED CRACKS, WIDE AT THE TOP, AND NARROW BELOW.
623
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