Where the water is deep, but having a sound bottom, a strong chest, called a caisson, must be formed, so that the sides may easily be disengaged from the bottom of the river, being bevelled where the pier is to be built, and the caisson pro perly placed over it, and kept in this situation by ropes: begin to build, and as the work advances it will sink gradually, and at the same time keep continually bracing the sides with timber, to prevent the ambient water from crushing it toge ther, and thereby not only spoiling the work, but drowning the workmen. When the pieris of such height as to be deeper than the water, the sides may be disen gaged, and the bottom of the caisson will remain under the pier, as a footing on which it is to rest : for this purpose the bottom of the caisson should be made very strong. Where the foundation is not firm, recourse must be had to piling, as in other such foundations.
With regard to the superstructure of a stone bridge, the arch stones sometimes terminate in a curve parallel to the in trados, and sometimes the joints of the arch stones are continued through the spandrils, observing to break joints side ways; at other times, the upper ends of the arch stones terminate so as to fit the beds and upright joints of every course of stone. The joints of the arch stones
are sometimes joggled with pl ugs,i n order to prevent them from passing each other. The piers are generally solid pieces of masonry from the foundation till they come to the spring, or above the spring of the arch ; thence arches, or complete cylindrical vaults, are sometimes thrown, in order to lighten the bridge, and brace every two adjacent arches between which they are placed. When the abutments are deep, and extend considerably along the road-way at each end, walls on each side of the road-way should be built, similar to those used in aquatic piers, and either strengthened with counter r vaulted under and across the wr.y. When there is a heavy pres sure of earth between the side of' the abutments, these sides should be both concave in any vertical, and also in any horizontal sections.
In stone bridges, when the extrados is a curve, and when the work' is coursed, the intersection of the bedding joint of every two courses on the face of the masonry ought to be parallel to the in tersection of the extrados with this face, as this position of the joints is not only more beautiful, but is also more agree able to the laws of strength, than those bedding joints which have their intersec tions in horizontal planes.