44 GENERAL SCIENCE 



sity. Our very existence depends on it. Our health 

 requires it. 



It is necessary to take into account the element of time 

 in calculating the amount of work done by a force. If 

 one horse can do a piece of work in one hour and it re- 

 quires three hours for another to do the same work, there 

 is a great difference in their rate of doing work. This 

 rate of doing work is called power. James Watt (1736- 

 1819), who invented the steam engine, thought that the 

 average horse could do 33,000 foot pounds of work per 

 minute; that is, raise 33,000 pounds one foot in one 

 minute or 550 pounds one foot in one second. While 

 this number is probably too high, it has been taken as 

 the unit of power in English-speaking countries and has 

 been named the horse power (H.P.). Steam engines 

 and motors are usually rated in horse power. 



Force of Expanding Gases. There are a number of 

 ways in which the force of expanding gas does work. 

 The steam engine is simply a device for utilizing the 

 energy of steam. When steam is produced under high 

 pressure and confined, it is potential energy, but when 

 it is allowed to expand in a cylinder, the energy is used 

 to produce motion which may be employed in various 

 ways (Figure 37). 



In the gasoline engine, the energy is set free by the 

 explosion of gasoline vapor and air. This explosion 

 produces a large volume of gases, the expansion of which 

 causes the piston to move, as in a steam engine. 



The efficiency of modern war engines, some of which 

 throw immense shells to a distance of twenty-five miles, 

 is due to the production of compounds which explode 

 with the evolution of large quantities of gases. With 

 these explosives it is necessary that the energy should 



