248 



MACHINERY IX CONXECTION WITH AIR. 



centre. The sails should, therefore, have a twist, to give 

 them the most perfect form, so that the parts nearest the 

 centre may form an angle of about 68 degrees with the 

 wind, the middle about 73 degrees, and the tips about 83 

 degrees. 



In order to produce the greatest effect, it is necessary to 

 give the sails a proper velocity as compared with the ve- 

 locity of the wind. If they were entirely unloaded, the ex- 

 tremities would move faster than the wind, in consequence 

 of its action on the other parts. The most useful effect is 

 produced when the ends move about as fast as the wind, 

 or about two-thirds the velocity of the average surface. 



The most useful wind is one that moves at the rate of 

 eight to twenty miles per hour, or with an average j^ress- 

 ure of about one pound on a square foot. In large wind- 

 mills, the sails must be lessened when the wind is stronger 



than this, to prevent the 

 arms from being broken ; 

 and if much stronger, it 

 is unsafe to spread any, 

 or to run them. 



The force of wind may 

 be usefully applied by al- 

 most every farmer, as it 

 is a universal agent, pos- 

 sessing in this respect 

 great advantages over 

 water-power, of which 

 very few farms enjoy 

 the privilege. 



Wind may be applied 

 to various jjurposes, such 

 as sawing wood by the 

 aid of a circular saw, 

 turning grindstones, and particularly in pumping water. 

 One of the simplest contrivances for pumping is represented 



Wirul-millfor pumping water on farms : 

 A, wind-mill ; ]i,vane; \, pump-rod. 



