LARY BRIDGE. 211 



foundations, previously prepared by the aid of the 

 diving bell. When the masonry was built a few feet 

 above the level of low water, the sides and ends of 

 these caissons were disengaged from the bottom, and 

 used in a similar operation for the next foundation. In 

 construction and application, these caissons differed 

 materially from any others with which I am acquainted. 



I have before stated, that the greatest difficulty in 

 the construction of a bridge of arches in this situation, 

 arose from the extreme looseness of the natural bed of 

 the river. It therefore became necessary, as the works 

 proceeded to adopt some effectual means of guarding 

 the foundations against the influence of the increased 

 velocity of the stream. An artificial bottoming quite 

 across the river at the site of the bridge, appeared the 

 only certain remedy, and it became a question what 

 were the best materials and mode of accomplishing our 

 purpose. 



Having ascertained from experiments that the strong 

 red clay, found in the limestone quarries in the neigh- 

 bourhood of the bridge, did not waste when exposed 

 to a current of water moving with a velocity of seven 

 feet per second, acting immediately on its surface, it 

 occurred to me, that this clay and the calcareous spar 

 obtained from the same source, would form the best 

 possible materials. As soon, therefore, as the bed of 

 the river had scoured away to the depth at which we 

 thought it expedient to commence this artificial bottom- 

 ing, the work was begun by depositing the above-men- 

 tioned clay, mixed with small stones, over the whole 

 surface of the bed of the river, from 60 feet above, to 

 70 feet below, the bridge ; which was proved to be the 

 extent of the increased current. The clay was kept, 

 as nearly as possible, about two feet thick, and lastly 

 it was covered with rough blocks of limestone and spar, 

 within one foot of the original bed of the river. The 

 stone being thus firmly imbedded in the clay, while the 

 clay (being impervious to the current) prevents the 

 water getting clown to, and acting on, the loose bottom ; 

 the whole is, by their united assistance, secured in a 



