ENCLOSING DAMS. 69 



ments, which had been brought close up on both the north and 

 south sides, began to push it inwards, distorting the circle by 

 unequal pressure, which had taken no effect as long as the water 

 was inside the coffer-dam. Some of the clay from the Zuider 

 Zee embankment was removed to the inside of the dam, but 

 as it still continued to move, the water was let in again ; not, 

 however, before it had bulged in on both sides. More sand 

 was then deposited inside the dam, where it had bulged, and 

 the water again pumped out. 



For letting the water into the dam, a syphon was adopted. 

 This was 16 inches in diameter, fitted with a foot valve outside, 

 and screw sluice-valve inside. 



At one time the dam was subjected to a different kind of 

 strain from any which it was calculated to resist. The water 

 outside, owing to a violent south-west wind, fell to about 8 feet 

 7 inches below datum, leaving a depth of nearly 7 feet inside the 

 dam. An attempt was made to reverse the syphon, but this being 

 unsuccessful the water bent the darn outwards, opening the 

 laminated waling, as well as the other walings at the joints, to 

 the extent generally of 2 or 3 inches, but no actual rupture 

 occurred sufficient to cause leakage. The openings in the 

 walings were afterwards filled up with blocks of wood, and when 

 the water was again pumped out, the outside pressure effectually 

 tightened the work. A second laminated waling of four half 

 timbers of Riga red pine was also added just above the original 

 laminated waling, and was secured with IJ-inch bolts to the 

 inside ring of piles only, in order to avoid increasing the number 

 of through-bolts. More sand was also deposited inside, until 

 the quantity reached some 33 cubic yards per lineal yard of 

 coffer-dam. 



Subsequently the water found its way into the dam, and 

 overpowered the pumps. It first made its appearance as a small 

 stream at the bottom of the pump well. By filling the hole with 

 straw, clay, and stones, and depositing sand against the dam on 

 the outside, the leak was stopped; but it soon burst out again in 

 the pump well, a little on one side of the spot where it first 

 appeared. To prevent disaster, two sluices were opened, and the 

 water let in. These sluices were about 4 feet long by 15 inches 

 deep, their tops being at the level of high water. The sheeting- 

 piles of the pump well were at once drawn, and a row of 

 sheeting-piles of whole timbers were driven across the bad place 



