or nearly four feet, for the depth at which the caisson will float. This leaves the sides one foot above the water surface.
Putting on a second course of masonry and three more side courses of timber, we have
| Floor | 21,000 | lbs. |
| Two courses masonry | 115,200 | lbs. |
| Six side courses | 15,480 | lbs. |
| In all | 151,680 | lbs. |
which divided by 62, and by 350, gives seven feet very nearly; leaving the top one foot above the surface.
In the same manner we proceed until the caisson grounds upon the bed, which has been previously prepared, either by pile-driving or by dredging. The bottom being reached, the sides are taken off, and the masonry remains upon the floor. The caisson may at any time be grounded by filling with water, and may be raised again by pumping out. The masonry may be laid either from barges or rafts at the site, or at the shore. Guide piles are necessary to insure the descent in the proper manner, and to prevent overturns.
In laying stone under water, it is to be remembered that masonry submerged loses 62
100 nearly of its weight, and is consequently more liable to be injured by shocks than when above the surface.
CHAPTER XIII.
SUPERSTRUCTURE.
283. Nothing aids more the proper accomplishment of any object than a correct idea of what is wanted. The following definition is given by Mr. W. B. Adams, of what good superstructure should be:—
“The principal requirements of permanent way are: That it shall be well drained, especially in contiguity to the substructure; that the weight and damaging power of the locomotives and rolling stock should be considered the data for calculation; that the strength, hardness, and tenacity of rails, and the immobility of the substructure should be adapted to the hardest work to which the railway is to be subjected; that the substructure should have an amount of bearing surface proportioned to the load to be borne, and the nature of the rail and ballast; and a sufficiently fair hold in the ground to prevent looseness or lateral motion, from the side lurches of the engines and trains; that the rails should possess so much vertical and lateral stiffness, either in themselves or in their fastenings, as to prevent all deflection; and have sufficient hardness of surface not to laminate or to disintegrate beneath the rolling loads; also, to have sufficient breadth or tread surface to diminish the crushing effect of the wheels.
“They should be as smooth as possible, to prevent concussion, and be laid at the proper angle, and the curves regularly bent, so as to insure the accurate tread of the wheels. The joints should be so made that the rails may practically become continuous bars, yet with freedom to contract and expand without being too loose. And with all this there should be interposed between the rails and the solid ground, some medium sufficiently elastic to absorb the effect of the blows of the wheels, without being crushed or forced down into the ballast, and yet stiff enough to keep the upper surface of the rails in a uniform plane.”