CONSTRUCTION OF BRIDGES. 273 



ingenuity in this respect that the magnificent bridge 

 over the Khine, near Strasbourg, was so rapidly con- 

 strutted. 



Each of the piers of this bridge rests on a founda- 

 tion composed of four iron caissons of large size and 

 weight. Each caisson was open at its lower part 

 (fig. 53). The upper part supported three shafts, 

 a middle and two lateral ones. All three rose above 

 the surface of the waters of the Rhine. The middle 

 shaft communicated wdth the open air, and the water 

 rose in it to the general level of the river. It enclosed 

 a dredging-apparatus worked by a steam-engine. 

 This dredge, as well as the shaft itself, descended to 

 the bottom of the river. The workmen loaded the 

 compartments of the dredge, which discharged its 

 load into the river. 



The two lateral shafts terminated at the- upper 

 part of the caisson. The workmen first shut them- 

 selves up hermetically in the upper part of the shaft, 

 which they afterwards put in communication with 

 blowing-machines. The compressed air driven by 

 these machines gradually expelle 1 the air from the 

 shaft, and ultimately from the caisson, into which the 

 workmen would now descend and commence their 

 excavations, carrying the debris to the dredges. 

 When the workmen wish to leave, they first mount 

 to the upper part of the lateral shaft ; the action of 



