COMPOUND MOTIOX. 19 



sustains a shock equal to the united forces of both. Two 

 men accidentally coming in contact, even if walking 

 moderately, receive each a severe blow; that is, if each 

 were walking three miles an hour, the shock would be the 

 same as if one at rest were struck by the other with a 

 velocity of six miles an hour. This principle accounts for 

 the destructive effects of two ships running foul of each 

 other at sea, or of the collision of two opposite trains on 

 a rail-road. 



The preceding principles show that a sledge, maul, or 

 axe will always strike more effective blows when made 

 heavier, if not rendered unwieldy. 



COMPOUND MOTION. 



It often liappens that two or more forces act on the 

 same body at the same time. If they all act in the same 

 direction, the effect will be equal to the sum of the forces 

 taken together ; but if they act in opposite directions, the 

 forces will tend to destroy each other. If two equal 

 forces act in contrary directions, they will be completely 

 neutralized, and no motion will be produced. Thus, as 

 an example of these forces a bird flying at the rate of 

 forty miles an hour, with a wind blowing forty miles an 

 hour, will be driven onward by these two combined forces 

 eighty miles an hour ; but if it undertake to fly against 

 such a wind, it will not advance at all, but remain station- 

 ary. A similar result will take place if a steam-boat, 

 liaving a speed of ten miles an hour, should first run 

 down a river with a current of equal volocity, and then 

 upward against the current; in the first case it would 

 move twenty miles an hour, and in the latter it would not 

 move at all. 



Where forces act neither in the same nor in opposite 

 directions, but obliquely, the result is found in the follow- 



