SUBMARINE WORK. 



653 



There is also sometimes danger that the bell may settle in the 

 soft mud, and be held there by suction. Such a case once oc 

 curred in New York harbor, when a party had gone in the bell 

 as a sort of pleasure excursion. The difficulty looked threat 

 ening, but one of the party proposed rocking the bell, ana 

 doing so the water was forced under, and the bell was lifted 

 from the ooze. 



FIXED APPARATUS SUPPLIED WITH COMPRESSED AIR. 



As the workmen cannot leave the bell, this difficulty if pos- 

 sible is obviated by moving the bell. Frequently, however, 

 submarine operations are to be carried on only in one spot, as 

 in building bridges, when the foundations of the piers are to be 

 laid, or in building breakwaters; laying the foundations oi 

 light-houses, or other similar work. In such cases, structures 

 which in principle are the same as the diving-bell, are fre- 

 quently employed. The oue which was used to build the piers 

 of the magnificent bridge over the Rhine, near Strasbourg is 

 represented in the cut. Each of the piers of this bridge rests 

 on a foundation composed of four large iron caissons, cf great 

 weight. Each ca^son was open at its lower end. The upper 



