ENERGY AND ITS TRANSFORMATIONS. 



15 



the method of its variation are often found useful. In 

 such diagrams the ordinate is usually made propor- 

 tional to the force acting, or to the resistance, while the 

 abscissas are made to measure the space traversed. 

 The curve then exhibits the relations of these two 

 quantities, and the enclosed area is a measure of the 

 work performed. With a constant resistance, the figure 

 is rectilinear and a parallelogram ; with variable veloci- 

 ties and resistances, it has a form characteristic of the 

 methods of operation of the part or of the machine 

 the action of which it illustrates. 



Power is defined as the rate of work, and is meas- 

 ured by the quantity of work performed in the unit 

 of time, as in foot-pounds or in kilogrammetres, per 

 minute or per second. The unit commonly employed 

 by engineers is the horse-power," which was defined 

 by Watt as 33,000 foot-pounds per minute, equivalent 

 to 550 per second, or 1,980,000 foot-pounds per hour. 

 This is considered to be very nearly the amount of 

 work performed by the very heavy draught-horses of 

 Great Britain ; but it considerably exceeds the power 

 of the average dray-horse of that and other countries, 

 for which 25,000 foot-pounds may be taken as a good 

 average amount. 



The metric horse-power, called by the French the 

 ckeval-vapeiir, or force de cheval, is about i J per cent, 

 less than the British, being 542.47 foot-pounds or 75 kilo- 

 grammetres per second, 4500 kilogrammetres per 

 minute, or 270,000 per hour. These quantities are al- 

 most invariably employed to measure the power ex- 

 pended and work done by machines."^ 



* Various nations have a standard "horse-power" derived from 

 Watt's, but, owing to differences in weights and measures, they are not 



