532 



Farm Mechanics. 



For wood sawing also the windmill may often be used 

 to advantage, by getting everything in readiness to do the 

 work on those days when the wind shall be strong, but for 

 this kind of work mills as large as 12 to 16 feet in diameter 

 are required. 



688. Wind Pressure The pressure which the wind may 

 exert upon a surface depends primarily upon (1) its 

 .weight per cubic foot, (2) its velocity, and (3) the angle 

 at which it strikes the surface. The weight of the wind 

 per cubic foot is greater when the air temperature is low 

 and when the barometric pressure is high ; this being true, 

 the capacity of a windmill in a given place varies with the 

 season, being greatest in winter and least in summer, for 

 like wind velocities. 



As the weight of a cubic foot of air decreases with alti- 

 tude windmills at sea level can do more work than those 

 at hights of 1,000, 2,000 or 3,000 feet, when the air tem- 

 peratures and wind velocities are equal. 



669. Relation of Wind Pressure to Wind Velocity 



When conditions are similar wind pressures increase as 

 the squares of the wind velocity. .Thus, if the wind pres- 

 sure at 5 miles per hour is taken as 1, then at 10, 15, 20, 

 25, 30, 35 and 40 miles per hour the wind pressure will 

 increase in the ratio of the squares of the numbers 2, 3, 4, 

 5, 6, Y, 8 ; that is to say, a 10 mile wind may exert 4 times 

 the pressure that a 5 mile wind does, and a 40 mile wind 

 a pressure 64 times as great. 



Taking the air at a pressure of 2,116.5 Ibs. per sq. ft. 

 the wind pressures at different velocities and temperatures 

 will be as stated in the table below: 



Table giving the pressure of the wind per sq.ft. at different 

 velocities and temperatures when the barometric pressure 

 remains the same. ( Wolff.) 



