CISTERNS. 89 



SOLUTION. 96 inches -f 72 inches = 168-7-2=84 inches, 

 mean diameter; then 5541.77, area of mean diameter, x 48 

 inches, depth, =266004.96 ^231 = 1151.53 gallons. Ans. 



ISToTE. The quantity of water which falls upon most 

 farm buildings is sufficient to afford an ample supply for 

 the domestic animals of the farm, when other supplies fail, 

 were cisterns large enough to hold it provided. The aver- 

 age amount of rain that falls in the latitude of the Northern 



O 



States during the year, is about 3 feet per year, or 3 inches 

 per month. Every inch in depth that falls upon a roof 

 yields 2 barrels for each ten feet square, and 72 barrels a 

 year are yielded by 3 feet of rain. A barn 30 by 40 feet 

 supplies annually from its roof 864 barrels, which is more 

 than 2 barrels per day, the year round. 



The size of cisterns should vary according to their in- 

 tended use. If they are to furnish a daily supply of water, 

 they need not be so large as for saving supplies against 

 summer and droughts. 



The size of the cistern in daily use need not exceed that 

 of a body of water on the whole roof of the building, 7 

 inches deep, or two months' greatest fall of rain. Cisterns 

 intended to save the water to draw from in time of drought, 

 should be about three times as large. 



To ascertain the size of cisterns adapted to roofs, <&c. 



RULE. Multiply the length of the roof in inches by the 

 breadth in inches, and that by the depth of the fall of rain 

 required to be saved, and divide the product by 231, and 



