LEGHORN DOCK. 



201 



It was then possible to remove in the dry the ashlar facing of 

 the circular head and floor. The masonry and concrete backing 

 was also removed, as far as it was safe to do so without risk 

 of the soil slipping. The hole was then filled up to within 6 

 feet 6 inches of the surface, to form the bed or platform on 

 which to construct the caisson. 



The form of this caisson was partly rectangular and partly 

 parabolic, corresponding to the outline of the new head of the 

 dock (Fig. 187); the length was 148 feet 2 inches, and the 

 width 101 feet, giving a total area of 1402 square yards. The 

 working chamber was divided into five compartments by means 

 of four principal equidistant transverse girders, armed with a 



ffO fl 10 



i^/^5*rS 



\SO 20 



Level of Sea 



FIG. 188. 



cutting edge, and placed in the same plane as the cutting edge 

 of the sides. These open girders were maintained in position 

 and strengthened by means of raking struts, presenting the form 

 of two brackets placed back to back. By this arrangement of 

 cross-girders the weight of the caisson was distributed over the 

 surface much more satisfactorily than if the lines of support 

 consisted only of the cutting edges of the sides of the caisson. 

 The arrangement also admitted of the ceiling of the working- 

 chamber being constructed of longitudinal beams 3 feet 3f 

 inches deep and 50 feet 6 inches long only, instead of trans- 

 verse beams exceeding 98 feet 6 inches in length. 



