IRRIGATION 229 



neath the water. It is operated by com- 

 pressed air, hence an air pump and an air 

 tank are required. Its chief advantage for 

 domestic purposes lies in the fact that it 

 starts automatically when the faucet is 

 opened, thus giving a supply of cold water 

 direct from the well. For greenhouse pur- 

 poses this is a disadvantage, as the water may 

 be too cold to use on the plants. 



Pump cylinders should not be more than 

 20 feet above the surface of the water, as this 

 is the limit of practical suction. When the 

 water, is more than 20 feet below the surface 

 the pumping cylinders are lowered accord- 

 ingly. In deep wells it is common to lower 

 the pumping cylinders well into the water. 



Capacity of Pumps. The capacity of a 

 pump depends upon the size of the cylinder 

 and the length and rapidity of the strokes. 

 The table on page 230 gives the discharge per 

 stroke in gallons, of pumps having cylinders 

 of various sizes. This, multiplied by the 

 number of strokes per minute, will give the 

 capacity per minute. 



Power Required. The power required to 

 operate a given pump may be determined as 

 follows: Multiply the number of gallons 

 pumped per minute by 8.357 pounds (the 



