SLIPWAYS. 



315 



correctness of the work during sinking, and before it is 

 permanently left. 



The following is a method of constructing slip foundations 

 under water, adopted and described by Mr. J. Thompson. 1 

 " When the portion of the site below low water had been 

 dredged out to the desired depth, the foundation was made by 

 tilling in broken stone of convenient size to near the level of the 

 intended platform, upon which a layer of macadam was placed, 

 bringing the foundation up to the required height. 



" As a guide for the accurate execution of this work, a line of 

 piles, A (Fig. 312), was driven on each side of the foundation 



Fia. 312. 



FIG. 313. 



clear of the sides of the timber platform, and to these piles, 

 guide-timbers, B (Fig. 310), were affixed at the required inclina- 

 tion of the slipway, and at the depth of the ends of the straight- 

 edge above it. 



" The foundation is now ready to be dressed off true by the 

 divers, who, as they frequently had to work in the dark, were 

 provided with iron-faced straight-edges (Fig. 312), made about 

 the same weight as a similar bulk of water, so as to be readily 

 moved. These straight-edges were long enough to reach across 

 the entire foundation, and to slide under the guide-timbers. 

 1 M.P.I.C.K, vol. Ixxii. p. 167. 



