CAISSONS. 



245 



curves of the sides are arcs of circles whose radii are 60 feet and 

 120 feet. 



The heel-post works on a steel pivot let into the heel-post 

 stone, and there is a roller fixed to each gate at a distance of 

 26 feet 8 inches from the heel-post, running on a cast-iron 

 roller-path, to assist in bearing the weight of the gate. 



The gates are secured at the top by strong iron anchors let 

 into the masonry. 



Caissons. Caissons may be considered as of two types, viz. 

 floating and sliding. Under the first may be included all those 

 which, whatever their shape, are raised from the bottom, and 

 floated out of their position when the entrance is opened. Under 



FIG. 237. 



FIG. 238. 



the second may be classed all those which, while still resting on 

 the bottom, are removed from the entrance by being hauled 

 endways into a chamber specially constructed for its reception, 

 over ways which have plane sliding surfaces only, or on rollers 

 fixed to the masonry or to the bottom of the caisson. 



An objection to rollers is, that being always submerged they 

 are difficult to get at in case of repairs being needed. If the 

 rollers are fixed to the bottom of the caisson, there is the 

 additional risk of the whole being brought up by any obstruction 

 falling on the pathway and getting under the rollers. On the 

 whole, it appears that less trouble is likely to be experienced 

 with a caisson which is fitted simply to slide over the ways than 



