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. [Footnote: 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.] 773. OF THE CAUSES OF FISSURES IN [THE WALLS OF] PUBLIC AND PRIVATE BUILDINGS. The walls give way in cracks, some of which are more or less vertical and others are oblique. The cracks which are in a vertical direction are caused by the joining of new walls, with old walls, whether straight or with indentations fitting on to those of the old wall; for, as these indentations cannot bear the too great weight of the wall added on to them, it is inevitable that they should break, and give way to the settling of the new wall, which will shrink one braccia in every ten, more or less, according to the greater or smaller quantity of mortar used between the stones of the masonry, and whether this mortar is more or less liquid. And observe, that the walls should always be built first and then faced with the stones intended to face them. For, if you do not proceed thus, since the wall settles more than the stone facing, the projections left on the sides of the wall must inevitably give way; because the stones used for facing the wall being larger than those over which they are laid, they will necessarily have less mortar laid between the joints, and consequently they settle less; and this cannot happen if the facing is added after the wall is dry. _a b_ the new wall, _c_ the old wall, which has already settled; and the part _a b_ settles afterwards, although _a_, being founded on _c_, the old wall, cannot possibly break, having a stable foundation on the old wall. But only the remainder _b_ of the new wall will break away, because it is built from top to bottom of the building; and the remainder of the new wall will overhang the gap above the wall that has sunk. 774. A new tower founded partly on old masonry. 775. OF STONES WHICH DISJOIN THEMSELVES FROM THEIR MORTAR. Stones laid in regular courses from bottom to top and built up with an equal quantity of mortar settle equally throughout, when the moisture that made the mortar soft evaporates. By what is said above it is proved that the small extent of the new wall between _A_ and _n_ will settle but little, in proportion to the extent of the same wall between _c_ and _d_. The proportion will in fact be that of the thinness of the mortar in relation to the number of courses or to the quantity of mortar laid between the stones above the different levels of the old wall.