206 DYNAMICAL GEOLOGY. 



below, which slopes 1 : 425 feet, or 12^ feet per mile. The discharge of water at sea level is 

 so large, although dependent solely on the rains, that the city of Brooklyn, containing 

 nearly a million inhabitants, has derived from it its supply of water through a series 

 of reservoirs, constructed a little above the sea level. The water-plane is not that of a 

 hard-pan layer. Its position has been determined by well-digging. Out of the 421- 

 inches of rain (snow included) which annually falls, nearly 40 per cent becomes ab- 

 sorbed and subterranean. The Brooklyn engineer, Mr. T. Weston, observes that these 

 subterranean waters supply the small streams of the surface with the chief part of their 

 water, and discharge a large amount into the sea ; and after a careful survey of a part 

 of this southern slope, east of Brooklyn, 73-64 square miles in area, he reported tliat the 

 water supply from the surface streams was, on an average, 22 per cent of the precipita- 

 tion, or 30,000,000 gallons a day ; that 15 per cent additional came out along the shores 

 of the bays ; and that at least 40,000,000 gallons per day might be obtained in reservoirs by 

 proper arrangements. Mr. Weston holds that the water-plane is the upper limit of a water 

 region which extends from this plane downward to and below the sea level, and that there 

 is no hard-pan layer underneath. Friction and capillarity in the sands give it its height. 



A coral island but ten feet high and a few hundred yards wide, and 

 consisting of coral rock up to the water level with coral sands above, 

 generally yields, on digging down to the surface of the coral rock, a 

 sufficient supply of water for its inhabitants, and all of it has come from 

 the rains. The fresh water, moreover, is sufficient to exclude, by its sea- 

 ward pressure, all ingress of salt water. If this is true on a coral island, 

 the subterranean waters derived from the rains over larger lands should 

 be very great. Moreover, the salt waters of the ocean do not penetrate 

 far into the basement of a continent. An island may receive sea water to 

 its center at some unascertained depth below the sea level ; but not so 

 a continent. 



2. Force of Jloio. — The force of the flow of subterranean waters is due 

 to gravity, like the flow of surface waters. There is everywhere hydrostatic 

 pressure, varying directly with the height of the supply, minus the loss by 

 friction and capillarity. The height may be that of the neighboring hills, 

 or of distant mountains, according to the range of the sloping rock-layers 

 along which the water descends. It may be that of lakes small or large, for 

 these bodies of water have the double duty of supplying above-ground and 

 under-ground streams. While the hydrostatic pressure varies with the height 

 of the water-supply, the extent of the region served by a single source will 

 depend on the area of that source. 



Professor Edward Orton, of Columbus, Ohio, has proved that the hydro- 

 static pressure in the Findlay oil-region, and also in Indiana, where the bor- 

 ings descend to the Trenton limestone, reaching it at various depths to 1000 

 feet or more below the surface, is determined by the ivaters of Lake Superior. 

 The level of the lake is 600 feet above tide level ; and by adding this height 

 to the number of feet at which the Trenton lies, in any case, below tide level, 

 and calculating the hydrostatic pressure on this basis, he has found tliat it cor- 

 responds closely with the actual gas pressure at each boring. He holds that 

 this hydrostatic pressure determines the gas pressure in other regions ; and 



