134 



LECTURE 



shut one door behind him before opening the other, which leads into 

 the interior of the cyhnder. 



All being made tight, air is pnrnped into the cylinder by an engine, 

 and the water forced downwards in the pile until it is all driven out 

 at the bottom; workmen then enter through the air-lock, which only 

 permits a small portion of the enclosed compressed air to escape as 

 the doors are opened, and descending to the bottom excavate the soil 

 under the edge of the pile and in the centre allowing it to sink down 

 gradually by its own weight. It is directed in a vertical position by 

 guides above. As the pile sinks, new cylinders are added to the top, 

 when necessary, and after the whole is down to a firm foundation the 

 interior is filled up with rough stone or concrete, the air-lock removed, 

 a permanent cap substituted, and the superstructure raised upon it. 



In loose, sandy bottoms these piles have been sunk by permitting 

 the air to rush out under the edge of the pile and by carrying out 

 with it the sand so as to make an annular channel into which the pile 

 settles by its own weight. 



Man}' bridges in England and on the continent, and some in this 

 country, have been built in this way with great success. One of the 

 most interesting examples is that of the new bridge over the Rhine 

 at Kehl, near Strasburg, where caisson piles were sunk to a depth of 

 sixty feet below the surface of the Avater. ^ 



Frequently the bed of the river, between masonry piers and for. 

 a short distance above and below them, is paved, planked, or covered 

 with loose stone, to prevent the current from undermining the 

 structure. 



The foundations having been completed, and the piers having been 

 built up to a certain level, the bottom of the beam of a truss bridge 

 and the spring of the arch of a stone bridge, preparations must be 

 made for adding the superstructure. 



The truss or beam may be either made on the shore and floated 

 and raised bodily into its position, as in the case of the water tubes 

 of the Britannia bridge, or it may be built in its place upon a scaf- 

 folding or false work erected to support it, as in the case of the land 

 tubes of the same bridge and of most of the truss bridges built in 

 this country. 



For a stone or cast-iron arch a frame called a centring must be 

 employed, as shown in Figure 60, the top of which has the form of the 



FiK. 60. 



inner side of the arch, so that when the stones are laid upon it they 



