Chapter X. Of Stone Bridges, and what ought to be observed in the building of them

MEN at first made bridges of wood, as being attentive to their present necessity only;

but since they have begun to have a regard for the immortality of their name, and when riches gave them spirit, and conveniency to do greater things, they began to build with stone, which is more durable, of greater expence, and of more glory to the builders. In these, four things ought to be considered; that is, the heads, made in the banks; ‘the pisa stars, that are sunk into the rivers; the arches, that are supported by the pilasters; and the pavement, which is made upon the arches. The heads of bridges ought to be made very firm and solid, since they not only serve to support the weight of the arches as the other pilasters do, but they keep the whole bridge united besides, and prevent the arches from opening; and therefore they must be made where the banks are of stone, or at least of a solid soil. And if banks thus made very firm by nature cannot be had, they must be made firm and strong by art; making there other pilasters and other arches, so that if the banks should be ruined by the water, the way to the bridge may not be interrupted.

The pilasters that are made for the breadth of the river, ought to be in number even; as well because we see that nature has produced all those things of this number, which being more than one, are to support any weight, as the legs of men, and all the other ani-mass can justify: as also because this same compartment is more agreeable to be looked at, and renders the work more firm; because the course of the river in the middle, (in which place it is naturally more rapid, as being farther from the banks) is free, and doth not damage the pilasters by continually shaking them. The pilasters ought to be so comparted, as to full in that part of the river where the stream is less rapid,

The greatest current of the waters, is where those things gather together that fivim upon it, which may easily be known at the increase of the river. Their foundations must be made in that time of the year when the waters are lowest, that is, in autumn: and if the bottom of the river be of stone, or of too, or of scaranto, which as have said (in the first book) is a sort of earth, that is partly stone, the foundations will be had without the fatigue of digging; because these sorts of bottoms are an excellent foundation of themselves. But if the bottom of the river be of gravel or sand, one must dig until the solid ground is found; and if that be difficult, some of the gravel or sand must be dug out, and then piles, made of oak, must be driven there, which, with the iron points that are made to them, must reach the solid and firm bottom.

To lay the foundations of the pilasters, one ought to inclose but one part of the river only, and build in that, that the water may have its course by the other part left open; and thus proceed from one part to another, The pilasters ought not to be thinner than the sixth part of the breadth of the arch; nor ordinarily thicker than the fourth. They must be made with large stones, which are to be joined together with cramps, and with iron or metal nails, that by such concatenations they may come to be all as of one piece. The fronts of the pilasters are commonly made angular, that is, that they have in their exwemity 2 rectangle; and some are also made sometimes semicircular, that they may cut the water, and that those things which are carried down by the impetuosity of the river, may, by striking against them, be thrown off from the pilasters, and pass through the middle of the arch.

The arches ought to be made firm and strong, and with large stones, which must be well joined together, that they may be able to resist the continual passing of carts, and support the weight, that occasionally may be conveyed over them. Those arches are very firm that are made semi-circular, because they bear upon the pilasters, and do not flock one another. But if by reason of the quality of the site, and the disposition of the pilasters, the semicircle should offend by reason of the too great height, making the ascent of the bridge difficult, the diminished must be made use of, by making arches, that have but the third part of their diameter in height; and, in such case, the foundations in the banks must be made very strong. The pavement of bridges must be made after the same manner, as the ways are paved, of which mention has been made before. Hence, as all that is to be observed in the building of stone bridges has been seen, it is time that we pass on to the designs.