OUTLINES OF NATURAL PHILOSOPHY. 



with the direction of the wind. Thus a circular 

 or rotatory motion is given to the sails. 



The sails are so constructed, as to have different in- 

 clinations to the plane of their motion at different 

 distances from the axis, greatest nearer the centre, 

 and least at their extremities. This is called the 

 weathering of the sails, and is done in order to 

 make the momentum of the wind nearly the same at 

 all different distances from the centre of motion. 



369. Supposing the sail of a windmill to be a 

 plane, and the inclination of that plane to the axis 

 or the direction of the wind to be i 9 the effect of 

 the wind to turn the sail, in a plane at right angles 

 to its axis, will be the greatest when cos i x sin i* 



1 



is a maximum, or cos i ~ - . 



V & 



This gives = 54 44', and therefore the inclination 

 of the sail to the plane of its motion, or what is 

 called the angle of weather, = 35 16'. This is true 

 only when the sail is at rest, or just beginning to 

 move. When the sail is in motion, and of course 

 near the extremities of the sail, where it moves faster, 

 the angle of weather must be less. MACLAURIN has 

 given a formula for this case, Fluxions, vol. u. 

 913, 914. 



MACLAURIN'S theorem makes the weather to vary 

 from 26 34', at the point of the sail nearest the 



centre, 



