226 THE POPULAR SCIENCE MONTHLY. 



be defective. It prevents the direct comprehension in the mind of 

 energy as being motion and nothing else ; it leaves unexplained how 

 a body perfectly at rest can come to move ; and further implies the 

 dissipation of energy (which I have treated in a previous number of 

 this journal) in a new phase, for, if all the actual energy in the uni- 

 verse were to become potential, all the real and positive motions which 

 constitute life might indefinitely cease. 



Let us examine, then, some cases of " potential " energy, and see 

 if they be not actual, although under a disguise ; so that the present 

 definition of the conservation of energy may be replaced by the more 

 intelligible statement that motion is constant ; that it is never abol- 

 ished for a time, nor absolutely suspended, all appearances to the con- 

 trary notwithstanding; and that so, all cases of energy are dynamic, 

 and no part of them static, as now currently held. 



For matter seemingly at rest may contain within itself motions as 

 real as those to which at another time it openly gives rise, w T hen it 

 radiates heat or attracts by magnetism. 



First of all, then, let us consider the case of a stone at a height, say 

 on the brow of a cliff, capable of falling at any time when slightly 

 pushed. 



Gravity is the one force, of all the forms of energy, whose relations 

 with others it is most difficult to imagine. 



Other forces affect each other most palpably : magnetism forsakes 

 a magnet when it is made white hot ; chemical affinity is most sensi- 

 tive to variations of temperature, and even in some cases to mechani- 

 cal tremor ; the transmission of electricity is favored by the cooling 

 of a conductor, and so on. 



Otherwise is it with gravity : a given mass of matter, however 

 mechanically moved, electrified, magnetized, heated, or subjected to 

 chemical changes, at the same point on the earth's surface, always 

 weighs the same. 



The only force with which gravity has any analogy is magnetism; 

 and were magnetism always attractive, instead of polar, with equal 

 opposite manifestations of attraction and repulsion, the analogy would 

 be a strong one. 



Let us, however, work out what analogy there is, and we may find 

 that as the subtile movements of light were made plain by the study 

 of the grosser movements of sound in air, so may the long-hidden 

 laws of gravity be revealed in part, by tracing the similitude existing 

 between their effects and those of common magnetism. 



Both forces obey the law of squares: according to that law, they 

 diminish as we recede from their centres of attraction. And what is 

 very suggestive is, that, as their appetites are satisfied, they decrease 

 in exactly the same ratio. 



Two small magnets at a great distance from each other (so as to 

 be practically out of the range of each other's influence), having each 



