

664 



DYNAMICAL GEOLOGY. 





most none ; cavernous limestones sometimes take in whole rivers ; 

 some sandy regions are always dry except when it rains. 



On the long southern side of Long Island the surface rises from the shore line at a 

 mean grade of 20 feet a mile for about six miles, and beneath the sloping surface, a 

 water level or plain (as proved by digging for wells which descend to it) commencing 

 at low-tide level, rises at an even grade of 12^ feet a mile. The material at and below 

 the surface is porous sand, more or less pebbly; and out of the 42? inches of rain (snow 

 included) which annually falls, nearly 40 per cent, become absorbed 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, west of 

 Brooklyn, 7364 square miles in area, he reported that the water supply from the surface 

 streams was, on an average, 22 per cent, of the precipitation, 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 plain is the upper limit of a water region which ex- 

 tends from this plain vertically downward to and below the sea-level, and that the 

 height and pitch of the water-plain is determined by friction from the sand, and not by 

 the existence of a "hard-pan " layer underneath. 



The water which has descended between inclined layers both nearly 

 or quite impervious, is like that in a long inclined tube, under hydro- 

 static pressure proportional to the vertical height of the source ; and 

 when a boring is made through the overlying beds to such a subterra- 

 nean stream the water rises and is sometimes thrown out in lofty jets. 

 In the annexed cut (Fig. 1094), a b represents a water-supporting layer, 

 b c the boring ; and c d the jet of water. The rise of the jet falls far 

 F] . 1094 short of the height of the source, 



because of the friction and the re- 

 sistance of the air. Such wells 

 are called Artesian wells, as they 

 were first made in the district of 

 Artois, in France. They are now 

 an important means of securing 

 water for irrigation and other pur- 

 poses in various parts of the world. 

 Even the dry plains of the Rocky 

 Mountain slopes and California have secured large supplies of water 

 by such means. They are seldom successful when the boring is made 

 into crystalline rocks ; but are often so when penetrating only the 

 stratified gravels of a region, there being usually in such formations 

 some hard-pan layer or clayey beds, which hold the water. 



The Artesian well of Grenelle, near the Hotel des Invalides, in Paris, is 2,000 feet 

 deep. At 1,798 feet, water was struck ; and it darted out to a height above the surface 

 of 112 feet, and at the rate of nearly one million of gallons a day. The pressure indi- 

 cated by the jet was equal to that of a column of water 2,612 feet high, or 1,160 pounds 

 to the square inch. Another, in the north of Paris, has been carried down to a depth 

 exceeding 2,000 feet, with a diameter of more than four feet to the bottom. All but 

 157 feet of it is below the sea-level. 



Section illustrating the origin of Artesian wells. 



