THE MEASUREMENT OF POWER 17 



piston and cylinder into mechanical energy for turning the 

 generator. The generator will produce heat, which will return 

 into the air, whence it originally came. Energy is never lost 

 nor destroyed, but at each step in the foregoing cycle, some 

 may be transformed into heat and escape without doing use- 

 ful work. When energy is expended by a certain agent, some 

 other material inevitably receives energy in the shape of work 

 or otherwise, the total being exactly equal to the energy stored 

 in the original source. 



The toy engine may in time produce many millions of foot- 

 pounds of work. The large engine in a central power plant 

 may do an equal amount with a few strokes of the piston. In 

 order to make a comparison of engines we must bring in the 

 element of time i. e. t the unit of power must take into 

 account not only the amount of work done but the rate of 

 doing it. The unit of power accepted in English-speaking 

 countries is the horsepower (h. p.), which represents a power 

 output of 550 foot-pounds of work per second, or 33,000 

 foot-pounds of work per minute. This might be done by 

 lifting a weight of 1100 pounds one foot in two seconds, or 275 

 pounds two feet in one second, or some such equivalent. 

 The product of force times distance, divided by time, will 

 always equal 550 foot-pounds per second, for each horsepower. 

 One horsepower exerted for the period of one hour gives rise 

 to another unit of work known as the horsepower hour (h. p.- 

 hr.) is frequently used in determining the fuel efficiency of a 

 motor. 



Every engine will waste a certain amount of energy in over- 

 coming friction within itself, besides the waste involved in 

 transforming energy from the chemical to the mechanical 

 state. Of the power generated in an engine cylinder, from 2 

 to 25 per cent, may be lost as compared with the work 

 which will be done at the flywheel. Of the latter amount 

 still another portion will be lost in a traction engine in over- 

 coming the friction in the transmission system, and in moving 



