MECHANICAL PARADOXES. 



sail C D, is intended to travel in the same 

 direction as the wind namely, from B towards 

 A, as indicated by the arrowheads. 



But the second and third arrangements 

 show an entirely different method, such as is 

 employed in windmills and turbines ; in which 

 the air is allowed to strike the sails obliquely, 

 because it is not itself moving in the desired 

 direction. If the boat E F has to travel to 

 the left, with the wind still blowing in the same 

 direction as before, the sail G H has to be set 

 obliquely. A portion of air, striking the sail 

 at G, and trying to go on to L, finds in this 

 sail an intervening obstacle, and cannot pro- 

 ceed without pushing it away. It cannot 

 push the sail straight on, in its own direction, 

 because the ship is set and steered broadside to 

 that direction, so as to be unable to travel freely 

 that way. It is only to the left that the ship can 

 travel easily, when its bow points to the left, and 

 therefore it is to the left that the sail must be 

 pushed away, in order to let the wind get past.| 



It is to permit the sail to be pushed to the 

 left that the wind must be allowed to strike 

 it on the right ; and since the wind, instead 

 of coming over the stern of the ship, comes 

 over its side, this can only be arranged by 

 setting the sails obliquely, in some such way 

 as is shown at G H. 



The speed of the ship will depend to some 

 extent on the degree of the obliquity at which 



IO 



