36 IRRIGATION. 



stream a thin shaving or other light floating substance is 

 thrown upon the surface, and the exact time in which it 

 moves over a definite distance, say 10 rods or 165 feet, is 

 carefully noted by walking along the bank watch in hand. 

 Let this time be supposed to be one minute. Then the 

 sectional area of the stream being 2 feet, this is multi- 

 plied by 165 and the product 330 is the number of cubic 

 feet of water passing down the stream in one minute. 

 A cubic foot of water weighs 62 1 | a pounds, therefore 330 

 cubic feet weighs 20,625 Ibs. If the dam is 4 feet high we 

 have 20,625 Ibs. of water per minute falling 4 feet, which 

 is equal to 82,500 Ibs. per minute falling one foot. This 

 would, as a matter of course, exactly balance the same 

 weight rising the same hight. The whole power of a horse 

 attached to suitable machinery is equal to that necessary 

 to raise 33,000 pounds one foot high in a minute. The 

 force exerted by the falling of 82,500 pounds in a minute 

 is equal to 2 1 | a horse-power. But a considerable allow- 

 ance must be made for friction, when waterwheels are 

 used, and especially where the fall is so small as here sup- 

 posed. It would not be safe to expect to gain more than 

 one half of the whole effect in this case. The power 

 gained would therefore, under ordinary circumstances, 

 be about T| 4 horse-power, or sufficient to raise about 

 40,000 Ibs. or 20,000 quarts a foot high per minute. This 

 is equal to about 11 quarts, 30 feet high, per second. 



If it is found necessary to store the water thus elevated 

 so as to extend the area that may be irrigated, cisterns of 

 substantial construction will be required. These should 

 be of brick or stone laid in cement, or hydraulic lime, 

 and strengthened with buttresses upon the outside. A bank 

 of earth should then be heaped up around it and sodded, 

 and if the bank be terraced, it may be utilized by plant- 

 ing it. A remarkably elegant structure of this kind is 

 to be seen in a market garden at Astoria, Long Island. 

 It consists of a large cistern of stone work surrounded by 



