I.] MATTEK AND ENERGY. 21 



Energy, like Matter, is measurable by quantity ; ^ and, Encrgy 

 like matter, energy is capable of being stored. A mill- ^^^IJ.J^^. 

 pond is a means of storing energy ; and a still better in- instance 

 stance, though exactly the same in principle, is Sir William ii3rciraulic 

 Armstrong's hydraulic accumulator. This is a contrivance aocumu- 

 for enabling a small steam-engine or other source of motive 

 power to do very heavy work for a short period of time. 

 It consists of a forcing-pump, through the action of which, 

 by hydraulic pressure, the motive power of the steam- 

 engine raises a very heavy weight. An amount of poten- 

 tial energy, proportionate to the weight multiplied into the 

 height through which it is raised, is thus accumulated ; 

 and when it is desired to give out energy and do work, this 

 is done by letting the weight descend and using its pres- 

 sure as motive power. The steam-engine is able to raise 

 the weight but slowly, but the weight is able to descend 

 rapidly, so that the potential energy which is stored up 

 through a comparatively long time may be given out in a 

 short time, and employed in work that requires a great 

 expenditure of energy in a short time, as, for instance, in 

 raising great weights. 



From the law that energy can be neither produced nor 

 destroyed, follows, to use the common expression, the im- Perpetual 

 possihility of a perpetual motion. But this is not a good 

 expression : in any obvious meaning of the words it would 

 be much nearer the truth to say that the law that energy 

 cannot be destroyed makes perpetual motion necessary ; 

 and this is rigidly true if, as we cannot doubt, heat is a 

 form of motion. What is meant, however, by the impos- in what 

 sibility of a perpetual motion, is the impossibility qf^^^l^. 

 an inexhaustible source of energy. It is impossible, for ^il^le- 



1 It may be thought that whatever is measurable at all must be 

 measurable by quantity. This, however, is not the case. Heat, for instance, 

 is measurable by quantity ; equal quantities of heat will melt equal quan- 

 tities of ice. But temperature is measurable, not by quantity, but by 

 degree : quantity of temperature would be an unmeaning expression. So of 

 force ; equal forces will balance, and will be balanced by, equal weights. 

 "We say, for instance, that the force of atmospheric pressure, as measured 

 by the height of the column of mercury that balances it in the barometer, 

 varies from hour to hour ; but the quantity of this force would be an ■ 

 unmeaning expression. 



