90 FERGUSON'S LECTURES. 



LECT. by the action of the wind upon four sails, every one 01 

 which ought (as is generally believed) to make an angle 

 of 54| degrees with a plane perpendicular to the axis 

 on which the arms are fixed for carrying them. It being 

 demonstrable, that when the sails are set to such an 

 angle, and the axis turned end-ways toward the wind, 

 the wind has the greatest power upon the sails. But 

 this angle answers only to the case of a vane or sail 

 just beginning to move :** for, when the vane has a cer- 

 tain degree of motion, it yields to the wind ; and then 

 that angle must be increased to give the wind its full 

 effect. 



Again, the increase of this angle should be different, 

 according to the different velocities from the axis to the 

 extremity of the vane. At the axis it should be 54 

 degrees, and thence continually decrease, giving the 

 vane a twist, and so causing all the ribs of the vane to 

 lie in different planes. 



Lastly, These ribs ought to decrease in length from 

 the axis to the extremity, giving the vane a curvilineal 

 form ; so that no part of the force of any one rib be 

 spent upon the rest, but all move on independent of each 

 other. All this is required to give the sails of a wind- 

 mill their true form : and we see both the twist and the 

 diminution of the ribs exemplified in the wings of birds. 



It is almost incredible to think with what velocity 

 the tips of the sails move when acted upon by a mode- 

 rate gale of wind. I have several times counted the 

 number of revolutions made by the sails in ten or fifteen 

 minutes ; and from the length of the arms from tip to 

 tip, have computed, that if a hoop of that diameter was 

 to run upon the ground with the same velocity that it 

 would move if put upon the sail-arms, it would go up- 

 wards of 30 miles in an hour. 



Note 37. See MACLAURIN'S Fluxions, eai the end. 



