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number of revolutions per second for a small one, taking for 
example a lift of 64’, Y2gh would be 64 and assuming that ¢ = 
1:25 the peripheral velocity would be 80’ requiring a fan 5’ in 
diameter to make 306 revolutions per minute, or else a fan 3’ 3 
-diameter making 510 revolutions per minute, to overcome this 
‘difficulty two ordinary pumps are often used, the first one dis- 
