WELLS, PUMPS, CISTERNS, FILTEBS. 117 



BUILD AND DIMENSION OF CISTERNS. 



In a stiff clay soil a small cistern of twenty to forty 

 barrels capacity might be safely cemented directly to the 

 earth, but in ordinary soils and for larger cisterns, a good 

 four-inch- wall of hard brick is on the whole the cheap- 

 est. It is important to make the excavation smooth, so 

 that the bricks can be pressed firmly against the earth ; 

 otherwise these will be pushed out and the cement cracked, 

 causing a leak. As to the dimensions, a cistern should 

 be about one-fourth deeper below the spring of the arch, 

 than its width inside. By this rule a cistern eight feet 

 wide will be ten feet deep below the arch. At the top is 

 a cast iron ring, twenty inches in diameter, for the man- 

 hole, covered with a tight fitting cast iron lid. The 

 ring has a flange two inches wide extending out over the 

 brick. The capacity of a cistern needed to save all the 

 water from a given extent of roof, will depend on the 

 total annual rainfall, its distribution throughout the 

 year, and the regularity with which it is used. A roof 

 ninety feet by twenty feet contains eighteen hundred 

 square feet. This is supposed to be the measure of the 

 building on the ground and not the shingled surface. In 

 the vicinity of New York the average annual rainfall is 

 about forty-two inches, or three and a half feet. This 

 would give sixty- three hundred cubic feet of water (1,800 

 ft. X 3y 2 =6,300). Since in that climate the rain is dis- 

 tributed pretty regularly through the year, it would only 

 be necessary to provide storage capacity for about one-third 

 of the rainfall of the year, or twenty-one hundred cubic 

 feet. This divided by four and one-fifth (the approxi- 

 mate number of cubic feet in a barrel of thirty-one and 

 a half gallons) gives five hundred barrels, and this quantity 

 of water demands a cistern, thirteen feet diameter, to be 

 nearly sixteen feet deep below the arch, or a square one, 

 thirteen feet across, to be nearly twelve and a half feet 



