44 NOTES ON DOCKS AND DOCK CONSTRUCTION. 



the top to the bottom of the caisson. A half-timber pile of 

 dimensions corresponding to this aperture was previously driven 

 and acted as a guide, which, with the assistance of a few 

 surrounding piles, maintained the caissons in their proper 

 position ; if the joint leaked, the wood swelled and prevented 

 any serious flow of water. 



The cast-iron grooves proving, however, too costly, were 

 abandoned, and the caissons were then held together longi- 

 tudinally by bolts passed through the angle-irons which were 

 fitted at the junctions. Four guide-piles were in this case 

 ordinarily driven so as to prevent the caisson swerving from 



FIG. 28. 



r Guide Piles 



Guide riles 



SECTION ON A B SHOWING GUIDE PILE &c. 

 FIG. 29. 



the vertical line during descent, and thus facilitated the opera- 

 tion of sinking. Besides these guide-piles, upright pieces of 

 timber cut to the shape of the caisson junction, and lined 

 between with felt, were placed on the inner side of the joints, 

 and timber struts, used inside for strengthening the caisson, 

 were abutted against them. Thus in proportion as the pressure 

 of water outside the dam increased the strain upon the struts, 

 the joint between the caissons became tightened. 



Half-section rings only were used in the upper part of the 

 dam; the convex sides facing the water, felted timber and struts 

 supported the end flanges on the inner side of the dam. A 

 frame of timber also stiffened their centre. 



