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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.
Arched cracks, wide at the top and narrow below are found in walled-up doors, which shrink more in their height than in their breadth, and in proportion as their height is greater than their width, and as the joints of the mortar are more numerous in the height than in the width.
The crack diminishes less in r o than in m n, in proportion as there is less material between r and o than between n and m.
Any crack made in a concave wall is wide below and narrow at the top; and this originates, as is here shown at b c d, in the side figure.
1. That which gets wet increases in proportion to the moisture it imbibes.
2. And a wet object shrinks, while drying, in proportion to the amount of moisture which evaporates from it.
79:401 : The text of this passage is reproduced in facsimile on Pl. CVI to the left. L. 36-40 are written inside the sketch No. 2. L. 41-46 are partly written over the sketch No. 3 to which they refer.