98 SMALL WATER SUPPLIES. 



umbrella fashion. A weight, however, prevents this 

 action taking place till the governor is travelling at 

 such a speed as to lift the weight. The mill may be 

 stopped by closing up the blades, independently of 

 the governor, by a chain gear at ground level, while 

 in other forms a band brake fulfils the same purpose. 

 On very large machines a small fan is fixed in a 

 plane at right angles to the main fan. When high 

 winds tend to drive the fan away from the wind, this 

 small fan starts to rotate and closes up some of the 

 blades. 



To find the approximate horse-power of a mill 



Let N be the number of blades, 



A the area of each blade in sq. ft., 



V the velocity of the wind in feet per second. 



NAV 3 , , 



H.p. = ,. . . . (29) 



1,660,000 



For instance, if N = 50, A = 2*5, and V = 30, 

 H _ 50 x 2-5 x 30 3 _ 337-5 _ 



1,660,000 1 66 

 just over 2 horse-power. 



A few words on the erection of pumps may 

 be an advantage to the reader. While it is usual 

 for the makers to build the pump and engine com- 

 plete, and erect and test it at the works, and finally 

 send one of their own fitters to erect it on the site, 

 it is usual for the purchaser to have the foundations 

 put in by an ordinary builder. For this purpose the 

 engine builders will supply a plan of the foundations, 

 which must be rigidly worked to according ..to the 

 dimensions entered thereon. Where the ground is 

 composed of sand, gravel, or hard clay, the load on the 

 foundations may be 2| tons per sq. ft., although 



