74 ELEMENTARY SCIENCE 



States can furnish about two hundred million horse-power, 

 while the amount utilized now is less than two million. 

 Rapid development of this water-power is being made. 

 Each year many factories, for the sake of economy, close 

 down their own steam-power plants and buy electric 

 power, which is delivered to them from plants like the one 

 at Keokuk. 



A horse-power is the standard unit in measuring the rate 

 of mechanical work. It is the amount of work done in 

 raising thirty-three thousand pounds one foot in one 

 minute. This unit was devised by Watt from observations 

 made on the amount of work done by strong dray-horses. 

 He found that the average dray-horse is able to work at 

 the rate of twenty-two thousand foot-pounds per minute, 

 foot-pound meaning the amount of work done in raising 

 one pound one foot. He arbitrarily increased this amount 

 by one-half, and used it to measure the power generated 

 by the steam-engine he had invented. It has been used 

 ever since. 



Water-Power and Gravity. You have noted that the 

 work that water does, or can be made to do, depends 

 upon the fact that it falls. Now before anything falls it 

 must be lifted. Then, when it falls, it falls with precisely 

 the same force as was required to lift it. Here we have a 

 statement of the law of gravity, by which we mean the ten- 

 dency of bodies to fall toward the center of the earth. (Gravi- 

 tation means the tendency of all bodies to attract each other. 

 Thus gravity, which is the most familiar example of gravi- 

 tation, is a case of gravitation in which the earth is the 

 principal body involved.) 



This law of gravity has been of great aid to man hi many 



