COMPOUND MOTION. 81 



more gradually slackened, and the passengers are not 

 pitched suddenly forward. A light wagon, rapidly 

 driven, possessing but little comparative force, is more 

 suddenly arrested, and the danger is greater. 



"When two bodies meet from opposite directions, each 

 sustains a shock equal to the united forces of both. 

 Two men accidentally striking, even if walking mod- 

 erately, 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 run- 

 ning 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 ax will always strike more effective blows when 

 made heavier, if not rendered unwieldy. 



SECTION" III. 



COMPOUND MOTION. 



It often happens 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 wiU 

 6e completely neutralized, and no motion will be pro- 

 duced. Thus, as an example of these forces — a bird 

 flying at the rate of forty miles an hour, ivith a wind 

 blowing forty miles an hour, will be driven onward by 

 these two combmed forces eighty miles an hour ; but 



