226 PNEUMATICS. 



per circle moves around with the wheel and vane on 

 the fixed lower circle, to which it is strongly secured so 

 as to admit of turning freely. In other respects it is 

 similar to the preceding. 



In all wind-mills, it is important that the sails should 

 have the right degree of inclination to the direction of 

 the wind. If they were to remain motionless, the angle 

 would be different from that in practice. They should 

 more nearly face the wind ; and as the ends of the sails 

 sweep round through a greater distance and faster, they 

 should present a flatter surface than the parts nearer 

 the centre. The sails should, therefore, have a twist 

 given them, so that the parts nearest the centre may 

 form an angle of about 68 degrees with the wind, the 

 middle about 72 degrees, and the tips about 83 degrees. 



In order to produce the greatest effect, it is necessa- 

 ry to give the sails a proper velocity as compared with 

 the velocity of the wind. If they were entirely un- 

 loaded, the extremities would move faster than the 

 wind, m 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 ve- 

 locity 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 

 pressure of about one pound on a square foot. In large 

 wind-miUs, 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. 



