42 FORCE AND MOTION. 



wind is twice resolved, and only one of the components 

 is of use in urging the boat forward. In Figure 15, 

 let KL represent the keel of the 

 boat ; BC, the position of the sail ; 

 and ^.7?, the direction and intensity 

 of the wind. In the first place, 

 when the wind strikes the sail thus 

 placed, it is resolved into two com- 

 ponents BC parallel to the sail, and 

 BD perpendicular to the sail. It is 

 evident that the first of these is of 

 no effect. But the boat does not move in the direction of 

 BD, which is, in turn, resolved by the action of the keel 

 and rudder into two forces, BL in the direction of the 

 keel, and BE perpendicular to it. The first of these pro- 

 duces the forward movement of the boat ; the second 

 produces a lateral pressure or tendency to drift, which is 

 more or less resisted by the build of the boat. 



93. The Third Law. Examples of the third law 

 of- motion are very common. When we strike an egg 

 upon a table, the reaction of the table breaks the egg; the 

 action of the egg may make a dent in the table. The re- 

 action of the air, when struck by the wings of a bird, 

 supports the bird if the action be greater than the weight. 

 The oarsman urges the water backward with the same 

 force that he urges his boat forward. In springing from 

 a boat to the shore, muscular action tends to drive the 

 boat adrift ; the reaction, to put the passenger ashore. 



94. Reaction in Noil- elastic Bodies. The 



effects of action and reaction are modified largely by 

 elasticity, but never so as to destroy their equality. Hang 



