16 POWER AND THE PLOW 



motion changed; or of tension, as when a spring is compressed 

 or extended beyond its normal state. 



In English-speaking countries the standard measure of 

 distance is the British standard yard, which is the distance at 

 a temperature of 62 F. between two marks on a certain bar 

 of bronze, deposited in the British office of the exchequer. 

 The usual measure of distance is one third of the yard or 

 one foot. 



If a force of one pound is exerted, either in lifting a pound 

 weight or in overcoming an equal resistance in a lateral direction 

 and this force be exerted through a distance of one foot, the 

 work done is the product of force times distance, or one foot- 

 pound. This is the common unit for the measurement of work, 

 though in* the same way we might have other units such as 

 "inch-pounds" or "foot- tons." The common term "ton- 

 mile " is not to be confused with these. The ton-mile is simply 

 the moving of a weight of one ton over the distance of one mile 

 on the surface, or the quivalent of this result, regardless of the 

 fact that more work will be required at one time to produce 

 a ton-mile than another, owing to the varying resistance offered 

 by different vehicles and road surfaces. 



Energy is the ability to do work. In compressing a spring 

 a certain force acts through a certain distance, the product 

 being a definite quantity of work. The spring becomes en- 

 dowed with energy, which has been stored up at the expense 

 of the force which compressed it, and is then able to do work on 

 another body. The crankshaft of an engine can be made to 

 revolve by means of the energy transmitted through the pis- 

 ton, and in turn will give off work to other masses. Work will 

 be required to lift water to a given height, but work can be 

 recovered from it as it flows to a lower level. Energy is the 

 equivalent of work in a potential state. Any form of energy 

 can be transformed into any other form. Thus, a fuel which 

 contains a chemical energy can be burned under a boiler to 

 produce heat energy. This heat may be transformed by a 



