ENGINEERING. 



257. 



No. 6 represents another variety of what 

 may be termed the square-block type of dam, 

 which appears to have been a favorite with 

 Moorish engineers. It is known as the Val 

 d'Inferno, and is in the province of Sarco, Spain. 



The dam at Quaker Bridge, N. Y., shown in 

 the diagram, towers above all other struct- 

 ures of the kind, actual or prospective. The 

 lake that will be created will include the pres- 

 ent reservoirs, covering the existing dams to a 

 depth of many feet. It will contain something 

 like 40,000,000,000 gallons of water, more or 

 less a matter of a few million gallons is of 

 small moment where such amounts are con- 

 cerned. It is estimated that the dam will im- 

 pound the whole rainfall of the Croton water- 

 shed, and will afford an ample supply for the 

 city, even if no rain at all falls for a period of 

 several months. The plans and calculations for 

 this vast structure were drawn by Benjamin S. 

 Church, chief engineer. The estimated strength 

 of the wall is about double what it will ever be 

 called upon to bear. The Croton water-shed, 

 as may be seen from the map, is an irregular 

 valley about 25 miles long by 12 or 15 miles 

 wide. 



The dam proper will be 1,350 feet long on 

 the crest, rather more than of a mile. It 

 will rest in a ditch, quarried out of the solid 

 rock, 216 feet wide at the widest part, and 

 will have an extreme height of 277 feet, de- 

 creasing to nothing at the wings. The dam 

 will not be laid on the arc of a circle, that plan 

 being regarded as obsolete for a structure of this 

 size, however it may add to the stability of 

 a short span like that at Furens. The dam is 

 planned to resist by sheer weight any possible 

 pressure that can result from the accumulation 

 of water. The idea that the extent of area in- 

 creases pressure is wholly erroneous. The press- 

 ure of water is due to its depth, not to its ex- 

 tent, as may be readily seen by reference to 

 any authority on hydrostatics. The danger 

 from a large impounded body of water arises, 

 not from the increased pressure upon the re- 

 straining dam, but from the cumulative rush 

 of water after a break has occurred. In an 

 earthwork dam, a trifling leak may spread with 

 disastrous rapidity and carry away the whole 

 structure ; but a masonry dam, if properly con- 

 structed, might be split from crest to base, and 

 would remain in position while the water 

 trickled through the crack. 



While the dam is in process of construction, 

 the natural overflow 

 of the Croton river 

 will be confined to 

 an artificial channel 

 running along the 

 side of the valley. 

 An unavoidable dan- 

 ger from freshets 

 must exist while the 

 work is going on, but 



the habits of the river have long been under 

 close scientific scrutiny, and with existing re- 

 VOL. xxvn. 17 A 



sources it is unlikely that serious interruptions 

 will occur. 



The system of construction is simple. The 

 first work is to lay bare the bed-rock of syenitic 

 gneiss that underlies the whole valley, at a 

 depth of about 90 feet below the river-bed, the 

 width of the trench corresponding with the 

 base of the dam. Two smaller trenches will be 

 made, each 10 feet wide and as many deep, 

 running lengthwise of the dam, and, after all 

 natural fissures in the bed-rock have been filled 

 with hydraulic cement, the trenches will be 

 built in with Cyclopean rubble, as it is termed 

 namely, large, rough, irregular blocks of stone, 

 laid so as to break joints and filled in with ce- 

 ment, so that the mass becomes as solid as 

 natural rock. All unevenness will be used to 

 anchor the dam beyond the possibility of slip- 

 ping. Gates and weirs of the best construc- 

 tion will be provided to carry off any overflow 

 during exceptionally wet seasons, and the 

 whole mass of water can be drawn off in case 

 of necessity. 



The estimates for the actual cost of con- 

 struction are $3,000,000, and the contracts 

 contemplate its completion in 1891. 



Floating Hock. An off-shore floating dock for 

 Cardiff, Wales, was constructed at Gray's, 

 near Tilbury, and towed to Cardiff in June. 

 In end elevation the structure resembles the 

 letter L, the horizontal limb, which is in fact 

 the pontoon, being wider than the vertical 

 limb is high. The upright part of the dock is 

 attached to vertical columns on shore by means 

 of booms arranged in pairs, so as to insure 

 parallel motion with the rising or sinking of 

 the dock. 



The pontoon, being filled with water, sinks 

 to the desired depth, and the vessel is floated 

 over it with obvious ease, eince it can be 

 warped into position broadside on. The keel- 

 blocks and bilge-blocks are all worked by ma- 

 chinery from the top deck, as in all the best 

 modern docks. The pontoon can be held at a 

 level or given an inclination, if a vessel with a 

 considerable list has to be taken up. 



Maritime Engineering. Among the great feats 

 of launching should be noted that of an enor- 

 mous lumber-raft at Joggins, Nova Scotia. 

 The construction was begun in 1885, the de- 

 sign being to tow the completed raft to New 

 York, and thereby save expense, and at the 

 same time bring to market larger logs than can 

 be handled on ordinary coasting-vessels. The 



Side. 



Croat Section on tkt Stoctt. 



THE GREAT RAFT. 



first attempt failed, the ways breaking down 

 under the enormous weight; 'tut the designer, 



