146 Mr Thomas Stevenson's Description of 



taken up, tlie fi'ames were, for the next compartment of 

 cutting, again superimposed upon one of the rows which had 

 been left standing in the last pit. In this way no rock could 

 possibly escape being removed ; and when the frames were 

 to be put down anew, there was no difficulty, (although the 

 pit was entirely covered with sand), in knowing exactly the 

 position which they were to occupy, as the piles which had 

 been left standing were an infallible guide. 



The advantages peculiar to this description of dam are 

 its cheapness, — its portability, — its ready adaptation to a 

 sloping, or even to a very ii'regular bottom, — the ease and 

 certainty with which the partitions between the different 

 pits are removed, and the double-framed walings that sup- 

 port and direct the driving of the piles. Wherever excava- 

 tions require to be made in a rocky beach, covered by a stra- 

 tum of sand, however thin, there need not be any hesita- 

 tion in adopting this form of dam, as there is no kind of 

 lateral support, such as stays or shores wanted, the structure 

 containing within itself the elements necessary for its stabi- 

 lity. It possesses, indeed, all the properties of a caisson, 

 and has the farther advantage of accommodating itself to an 

 irregular bottom.* 



I may observe, in conclusion, that although this form of 

 construction is specially adapted to marine works, in the exe- 

 cution of which it has proved a most valuable auxiliary, the 

 same principle might also be earned to a greater extent, and 

 be rendered fit, with little trouble, to answer for a variety of 

 works, — such as underfooting quay walls, founding bridges, 

 and in removing fords or other obstructions from the beds 



piles that were to rerciain, which, when the works were done, was cleared out 

 by means of a water-scour provided for the purpose of keeping permanently 

 open the navigable tract. 



* In situations also, where there is a considerable depth of water, and 

 where, consequently, the frames must be made so as to stand high above the 

 ground, it will be found of great advantage to plank the outside of the frames 

 between A and G. This will not only make the dam more water-tight, but 

 have the effect of binding and strengthening the framework. 



