Energy and Heat 121 



_4^ foot-pounds per second. Rate of working is termed 

 "Power", and, as indicated above, is measured by 

 dividing the work done by the time taken to do it. 

 The unit of power in use in Engineering is the " Horse- 

 Power", and an engine or other body capable of doing 

 work is said to be of 1 Horse-Power when it can perform 

 work at the rate of 550 ft.-lbs. per second. The 

 corresponding unit in the Metric System, the kilowatt, 

 is about f H.P. 



Exercise for Student. 



A ton of coals is raised from the hold of a ship to the wharfside 

 in 25 seconds. If the bottom of the hold is 18 ft. below the side 

 of the wharf, how many horse-power is required to raise the coal ? 



77. A man sitting still in a chair is doing no external 

 work, yet we know that he is capable of doing it under 

 suitable conditions. An engine with steam up is in 

 similar case. Both have a capacity for working, the 

 man by virtue of his muscular development, the engine 

 through the expansive force of the steam produced in 

 its boiler. The name given to the capacity for doing 

 work is "Energy", and the man and the engine are 

 said to possess a store of energy. It is only by the 

 expenditure of some part or all of that store that 

 either can perform work. 



It is usual to distinguish energy as being of two 

 kinds. Thus our 14 Ib. weight, when on top of the 

 table, possesses energy by virtue of its position. During 

 its fall to the floor, it may be caused to do work by 

 attaching a cord to it, the other end of which is con- 

 nected with clockwork whereby a fan may be rotated, 

 or other form of work done. The work done by 

 gravity on the weight overcomes the resistance of the 

 moving parts of the mechanism, and work is thus 



