68 NOTES ON DOCKS AND DOCK CONSTRUCTION. 



in the centre of the area to be enclosed. At the commencement, 

 some difficulty was experienced in getting the piles of the 

 exterior ring set out to a true circle, which was important. At 

 first a hempen rope previously strained was fastened to the 

 central dolphin, but the rope stretching irregularly the pi Irs 

 were not true to a circle. Floating rods were then tried, but 

 the wind and the tide deflected them sideways. At last a light 

 iron wire rope about inch in diameter, kept strained by a 

 weight, was used ; for a few days it gave irregular results, but 

 soon acquired its maximum stretch, and became sufficiently 

 accurate. It was found that if the guide-piles were set out 

 in this manner as close as 10 feet apart the resulting curve 

 was not very regular, they were, therefore, spaced 40 feet 

 apart. Three pile-engines were then set to the curve on a 

 radial lighter 40 feet broad at one end and 20 feet at the other. 

 These engines drove three piles equidistant in the 40-foot space. 

 The three pile-engines were driven by an 8 horse-power engine 

 with triple-purchase gear on to a long shaft. On this shaft 

 were three drums, each of which could be thrown out of gear 

 at any moment. 



In order to drive the sheet-piles, two other lighters were 

 provided, with one pile -engine on each, driven by an Appleby 

 steam crab. In every bay, the last pile but one was wider at 

 the base, the last pile being as usual a wedge or key to close 

 the other piles up. For preparing the piles, one circular- 

 saw table, driven by a 14 horse-power engine, just kept the 

 work going. 



The ground was of a very variable character; many of the 

 piles could be driven in about 10 minutes, whilst others could 

 not be got down without being split, and some could only 

 be kept down by letting the monkey rest upon them after the 

 blow had been struck, as they rose 18 or 20 inches if the 

 monkey was lifted immediately after the blow. It was sub- 

 sequently found that this occurred where the la} r er of peat was 

 the thickest and the substratum of sandy clay the softest. The 

 weight of the monkey used was one ton. 



To prevent the ground being scoured on the south side, 

 where there was at one time an extraordinary current, due to 

 the narrowing of the lake by the Zuider Zee dam, long mat- 

 tresses of fascine work were sunk against the coffer-dam. 



As soon as the coffer-dam had been pumped out, the embank- 



