34 IRRIGATION. 



is the cheapest kind of tank that can be made. One 16 

 feet square and 10 feet deep will contain nearly 20,000 

 gallons. Tanks of this character can only serve for small 

 gardens, or to store water which is pumped at night for 

 use during the day time. Either of these tanks, if filled 

 during the night (to do which will require a stream from 

 a pipe of an inch and a half in diameter constantly run 

 ning), and replenished during the day, will furnish 

 enough water to give more than one inch in depth over 

 an acre of surface. This is the least quantity that could 

 be depended upon in a dry season for any effective pur 

 pose, and would need repeating after an interval of four to 

 seven days, so that the maximum effort of a tank of this 

 size, with a well, windmill or horse-power attached, would 

 suffice only in an emergency to water four to seven acres 

 of land. Where the ground to be irrigated is of larger 

 extent, the tank room and water supply must be enlarged, 

 or the diameter of the pipe and power increased. The 

 capacity of the pipe increases as the square of the diame 

 ter, by which is meant that if the diameter is doubled 

 the capacity is quadrupled. Thus if a pipe one inch in 

 diameter supplies one quart per second, a pipe of two 

 inches diameter will furnish four quarts per second (or 

 two multiplied by two), and a pipe three inches diameter 

 will yield nine quarts (or three multiplied by three), per 

 second. At the same time the power must be increased 

 in proportion to the amount of water elevated, or disap 

 pointment will result. In estimating power a large allow 

 ance must be made for loss. A horse working in a rail 

 way-power can only raise an equivalent of three-fourths 

 of his weight ; the rest disappears in friction; and when 

 a stream of water is forced through a pipe of small di 

 ameter for a considerable distance, the loss of power in 

 friction is very large, and another fourth of the horse s 

 effort must generally be allowed to compensate for it. 

 One horse may be expected to raise 180 quarts one foot 



