METHODS OF CONSTRUCTING BREAKWATERS. 255 



When it is required to drop blocks or deposit them irregu- 

 larly, as in pell-mell work, whereby the withdrawal of T-bars or 

 lewises would be rendered difficult, one or other of the methods 

 represented by Figs. 73-78 may be adopted. That illustrated by 

 Fig. 73 was used in depositing the blocks forming the Alexandria 

 breakwater ; and that illustrated by Fig. 74 is now being used 

 for placing the wave-breaker blocks in the Madras breakwaters. 



Concrete blocks are usually made upon a level floor in some" 

 convenient situation, and when required for setting in the work, 

 they are loaded upon trucks by means of an overhead traveller 

 or " Goliath." On arriving at the breakwater, they are lifted by 

 the setting machine and placed in the work. 



Diving. The preparation and levelling of the beds, and the 

 setting of blocks below low water, necessarily requires the aid of 

 divers. Some engineers prefer the diving-bell to the helmet for 

 under- water work ; but when circumstances will admit of helmet 

 divers being employed, more work can as a rule be done in a 

 given time than when a diving-bell is used. 



At the Dover (Admiralty) pier, the number of blocks 

 (averaging 6 to 7 tons each) set in a " shift " of 4 hours by 

 means of the diving-bell was four, or at the most five. By the 

 introduction of diving dresses and helmets, the number was 

 increased to 8. Where a strong current existed, however, the 

 bell was found to have an advantage over the dress; indeed, 

 work could be executed from a bell under conditions where a 

 diver in a dress could not stand. When the velocity of a current 

 exceeds three miles per hour, helmet diving becomes very 

 difficult, and the strain on the life-line and air-pipe is excessive. 



At the Alderney harbour works the use of diving-bells was 

 given up, and helmet divers substituted, the reason assigned 

 being that when the bell was lifted a little off the bottom, the 

 rapid current drove the men against the sides of it. 1 It would 

 appear, however, that the conditions at Alderney must have 

 been exceptional, inasmuch as the experience there seems to 

 have differed from that at most other places. It is difficult to 

 understand how helmet divers, with their whole bodies exposed 

 to the force of the current, could have worked better than men 

 who were to a great extent protected from it by the bell. 



Over-end Tipping. Mound breakwaters are sometimes made 

 by tipping the material over-end in the same way that a railway 

 1 Min. Froc. Inst. C.E., vol. xxxvii. p, 361. 



