400 NOTES ON DOCKS AND DOCK CONSTRUCTION. 



fourth of the distance through which they had been depressed. 

 The discs being 2 feet in diameter, the pressure per square foot 

 was 2'50 to 318 tons in the first case, and 4-45 to 477 tons per 

 foot superficial in the second case. 1 



In timbering the sides of the trenches in sand and peat, 

 required in carrying out waterworks at the Hague, the method 

 usually adopted was to attach to the poling-planks or piles a 

 small iron pipe, which was connected by means of a flexible hose 

 to a hand force-pump. As the water issued from the lower 

 end, washing away the sand, the poling-plank sunk into its 

 place. In this way, two men could place from twelve to fifteen 

 planks, 8 feet long, per hour. With the view of saving time 

 and cost, the engineer, Mr. Waldrop, devised an improvement 

 which proved very satisfactory. An iron pipe, with holes drilled 

 in two rows opposite to each other, was, instead of being 



attached to the planks, 

 rcc p w ,,f> forced vertically into the 

 sand, and connected with 

 a powerful force-pump. 

 The pipe, once down, was 

 dragged along, cutting a 

 trench in the sand in its 

 course. In this trench, as 

 soon as made, the planks 

 were placed (Fig. 353). By 

 this arrangement, four men 

 were able to place from 80 

 to 100 planks per hour. 2 

 In placing piles in the open sea for defence purposes, the 

 American engineers adopted the following course : The site was 

 the outlet to the Bay of Mobile, exposed to the heavy swell of the 

 Gulf of Mexico, and through which there was a current of about 

 three miles per hour. The depth of water was about 8 feet, and the 

 mean rise and fall of tide 18 inches. A careful survey showed 

 that the bottom generally consisted of sand to 20 feet deep. 



Two rows of piles were driven 10 feet apart, the piles in one 

 row being placed opposite the intervals in the other. 



A steam fire-engine placed on the deck of a steamer, and 

 connected with the vessel's boiler, supplied the necessary watn- 

 pressure. 



1 MP.I.CR, vol. 1. p. 107. ' Engineering, vol. xlvii. p 250. 



Sand 



Sand 



Fro. 353. 



