266 



HARBOUR CONSTRUCTION. 



levelled beds of mass-concrete under water not being considered 

 practicable. 



In view of the advantages which would accrue if some means 

 of doing such work could be discovered, I gave the subject very 

 careful attention, and succeeded in devising a plan which has 

 proved quite satisfactory in practice. It has already been 

 referred to, but I will now describe it at greater length, and, in 

 doing so, will quote from an account of it which was given in 

 Engineering of September 16, 1892. 



" The method may be thus described : Let ab. Fig. 85, repre- 

 sent the level at which it is required to found the lowermost course 

 of blocks, at any convenient depth below low water. Careful longitu- 

 dinal sections are taken along the lines of the inner and outer 

 faces of the foundation, as at c, or, if it should be a wide one, upon 

 intermediate lines also, and these are plotted, full size, upon a 



FIG. 85. CROSS SECTION. 



Rail Straiglit edge. 



FIG. 86. ELEVATION OF PLANK 



SHOWING BOARDING &C. 



- 1-ROC.K' ( 



Mass-concrete foundation for under- water work. 



floor. Planks are then cut and made up by nailing on boarding, 

 where required, to correspond with the irregularities of the bottom, 

 as represented in Fig. 86. These are weighted with iron to the extent 

 necessary to make them sink, and they are fixed by means of bolts, 

 from 2 feet to 3 feet in length, let into the rock, as shown, and 

 secured by iron wedges. Wedges are used in preference to making 

 the bolts in the form of lewises, in order that they may the more 

 easily be withdrawn for use in the succeeding lengths of foundation. 

 " The planks are fixed by divers, and in moderate depths their 

 level is determined by means of an ordinary field-level and staff, the 

 latter being provided, if necessary, with a 10-feet or 15-feet lengthen- 

 ing piece. A diver holds the foot of the staff upon the top of the 

 plank, which is raised or lowered according to signal. It is thus 

 fixed at its correct level with perfect ease and accuracy. If found to 

 be more convenient, the divers may themselves adjust the plank by 



