I N 



IV. Wind, velocity of. 



The following- Table, drawn up by Mr Smeaton, will give the reader 

 a pretty precise idea of the velocity of the wind in different circum. 

 stances. (Phil. Trans. 1757.; 



WINDMILL. (Play fair. ) 



1. The impulse of a stream of air, striking with a velocity of t? feet per 

 second, on a plane whose area in feet =: a%, inclined at an angle to tha 

 direction of the stream, is in avoirdupois pounds, 



t> 2 a* sin. 2 



140 



2. The sails of windmills are so constructed as to have different incli- 

 nations to the plane of their motions at different distances from the axis ; 

 greatest nearer the centre, and least at their extremities. This is done 

 in order to make the momentum of the wind nearly the same as all dif- 

 ferent distances from the centre of motion. 



3. Supposing the sail of a windmill to be a plane, inclined to the axis 

 at an angle 8, the effect of the wind to turn the sail in a plane, at right 

 angles to its axis, will be the greatest when cos. 6 X sin. 2 is a maxi- 

 mum, or when cos. 6 = %. 



This gives = 54 a 44', and therefore the inclination of the sail to the 

 plane of its motion, or what is called the angle of u-eaiher, is 35 10'. 

 This is true only when the sail is at rest or just beginning to niom 

 352 



