212 HYDRODYNAMICS. 



DETERMINING THEIR SIZE. 



The size of cisterns should vary according to their 

 intended use. If they are to furnish a daily supply 

 of water, they need not be so large as for keeping sup- 

 plies for summer only. The average depth of rain 

 which falls in this latitude, although varying consid- 

 erably with season and locality, rarely exceeds seven 

 inches for two months. The size of the cistern, there- 

 fore, in daily use, need never exceed that of a body of 

 water on the whole roof of the building seven inches 

 deep. To ascertain the amount of this, multiply the 

 length by the breadth of the building, reduce this to 

 inches, and divide the product by 231, and the quotient 

 will be gallons for each inch of depth. Multiplying by 

 7 will give the full amount for two months' rain fall- 

 ing upon the roof. Divide by 31 J, the quotient will 

 be barrels. This will be about fourteen barrels for 

 every surface of roof ten feet square when measured 

 horizontally. Therefore, a cistern for a barn 30 by 40 

 feet should hold 168 barrels ; that is, as large as one 

 ten feet in diameter and nine feet deep. Such a cis- 

 tern would supply, with only thirty inches of rain year- 

 ly, no less than 630 barrels, or nearly two a day. 



Cisterns intended only for drawing from in times of 

 drought, to hold all the water that may fall, should be 

 about tliree times the preceding capacity. 



