MEANS OF TRANSPORT. 



the sails are set. Suppose, for the present, 

 that the air and the sails are absolutely free 

 from friction, so that they slip past each other 

 without any loss of power. Suppose, also, 

 that the ship and the water are equally friction- 

 less, and that no power is spent in lifting the 

 water aside. Then the wind, in going from 

 G straight on to L, will easily thrust aside the 

 sail to the left far enough to make the way 

 clear. How far this is, depends upon the 

 obliquity of the sail. 



When it is set as in the illustration, with 

 the distance G L equal to G K or H L, the 

 sail has to be pushed to the left from G H 

 to K L in order to leave the way clear for the 

 wind from G to get through past the corner 

 of the sail at L ; that is to say, the ship moves 

 to the left as far as the wind moves forward 

 in the same time. In other words, the boat 

 is travelling as fast as the wind. This is the 

 case when the boat is sailing straight across the 

 wind, and the sails are set at an angle of 45. 



But if the sails are set squarer to the wind, 

 as at OP, they have to travel further in order 

 to leave the way clear for the wind to go straight 

 on. Thus, while the wind goes from O to R, 

 the sail in front of it has to go from O P to 

 Q R ; and if the distance O Q or P R is twice 

 the distance O R, the ship has to go twice 

 as far as the wind goes in the same time ; that 

 is, it travels twice as fast. 



