82 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK i. 



the tube and that which the outer air opposes to it, necessarily 

 reduces the height. 



Nero's fountain is not a simple physical curiosity, and that is the 

 reason we have mentioned it here. The arrangement has been 

 reproduced and the principle applied in the construction of draining 

 machines, such as the machines in the Schemnitz mines in Hungary, 

 which are only gigantic Nero's fountains, constructed, it must be 

 understood, with the solidity necessary to an application of this kind. 



IV. USE OF COMPRESSED AIR IN BRIDGE BUILDING. 



Compressed air has also received an application of another kind 

 which is not less interesting than those to which we have just referred. 

 It has been used to force the water from metal caissons, intended to 

 form the foundations of the piers of bridges. We are indebted to 

 M. Triger, a French engineer, for the first idea and application of the 

 first method of this kind. Different processes have been used 

 according to the circumstances and the views of engineers who have 

 applied it ; but as the physical principle is the same, it will be suffi- 

 cient to describe one of them briefly in order to understand the 

 others. Let us take the one adopted in the construction of the bridge 

 of Kehl on the Ehine. 



Figure 53 represents the arrangements made for laying one of 

 these foundations, the interior of one of the caissons lowered below 

 the bed of the river, and the workmen who are clearing it. 



Let us imagine an enormous box, with sides solidly bolted and 

 strengthened with girders and iron supports in the interior as well 

 as on the upper side. This box, of rectangular form, is open at 

 the bottom, whilst the roof, pierced with tlnee circular holes, is sur- 

 mounted by three chimneys in iron plate, the two lateral chimneys 

 communicating "simply with the interior of the box, and each sur- 

 mounted with an air-chamber ; that in the middle descends below the 

 base of the box. Let us suppose this sort of diving-bell lowered 

 to the bottom of the river, so that its open base rests on the bottom : 

 the water will fill it, and, by virtue of the law of equilibrium of 

 liquids in communicating vessels, it will ascend in the three chimneys 

 to the level of the water of the river. If now, by using steam 



