When a dome or a half-dome is crushed from above by an excess of weight the vault will give way, forming a crack which diminishes towards the top and is wide below, narrow on the inner side and wide outside; as is the case with the outer husk of a pomegranate, divided into many parts lengthwise; for the more it is pressed in the direction of its length, that part of the joints will open most, which is most distant from the cause of the pressure; and for that reason the arches of the vaults of any apse should never be more loaded than the arches of the principal building. Because that which weighs most, presses most on the parts below, and they sink into the foundations; but this cannot happen to lighter structures like the said apses. [Footnote: The figure on Pl. CV, No. 4 belongs to the first paragraph of this passage, lines 1-14; fig. 5 is sketched by the side of lines l5--and following. The sketch below of a pomegranate refers to line 22. The drawing fig. 6 is, in the original, over line 37 and fig. 7 over line 54.] Which of these two cubes will shrink the more uniformly: the cube _A_ resting on the pavement, or the cube _b_ suspended in the air, when both cubes are equal in weight and bulk, and of clay mixed with equal quantities of water? The cube placed on the pavement diminishes more in height than in breadth, which the cube above, hanging in the air, cannot do. Thus it is proved. The cube shown above is better shown here below. The final result of the two cylinders of damp clay that is _a_ and _b_ will be the pyramidal figures below _c_ and _d_. This is proved thus: The cylinder _a_ resting on block of stone being made of clay mixed with a great deal of water will sink by its weight, which presses on its base, and in proportion as it settles and spreads all the parts will be somewhat nearer to the base because that is charged with the whole weight. III. ON THE NATURE OF THE ARCH. 779. WHAT IS AN ARCH? The arch is nothing else than a force originated by two weaknesses, for the arch in buildings is composed of two segments of a circle, each of which being very weak in itself tends to fall; but as each opposes this tendency in the other, the two weaknesses combine to form one strength. OF THE KIND OF PRESSURE IN ARCHES. As the arch is a composite force it remains in equilibrium because the thrust is equal from both sides; and if one of the segments weighs more than the other the stability is lost, because the greater pressure will outweigh the lesser. OF DISTRIBUTING THE PRESSURE ABOVE AN ARCH. Next to giving the segments of the circle equal weight it is necessary to load them equally, or you will fall into the same defect as before. WHERE AN ARCH BREAKS. An arch breaks at the part which lies below half way from the centre. SECOND RUPTURE OF THE ARCH.