238 MACmNERT IN CONNECTION WITH WATEE. 



31^, and the result will be the number of barrels the cis- 

 tern will hold. 



For each foot in depth, the number of l^arrels answer- 

 ing to the different diameters are, 



For 5 feet diameter 4.66 barrels. 



6 " 6.71 " 



7 " 9.13 " 



8 " 11.93 " 



9 " 15.10 " 



10 " 18.65 " 



By the rule above given, the contents of barn-yard 

 cisterns and manure tanks maybe easily calculated for any 

 size whatever. 



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 keeping supplies 

 for summer only. The average depth of rain which falls 

 in this latitude, although varying considerably with season 

 and locality, rarely exceeds seven inches for two months. 

 The size of the cistern, therefore, 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, divide the product by 231, and the 

 quotient will be gallons for each inch of depth. Multiply- 

 ing by 7 will give the full amount for two months' rain 

 falling upon the roof. Divide by 31|-, and the quotient 

 will be barrels. This will be about fourteen barrels for 

 every surface of roof ten feet square when measured hori- 

 zontally. Therefore, a cistern for a barn 30 by 40 feet 

 should hold one hundred and sixty-eight barrels ; that is, 

 as large as one ten feet in diameter, and nine feet deep. 

 Such a cistern would supply, with only thirty inches of 

 rain yearly, no less than six hundred and thirty 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 have 

 about three times the preceding capacity. 



