232 KINETIC THEORIES OF GRAVITATION. 



servatiou requires that all the successive spheres described by increas- 

 ing radii of action shall represent precisely the same amount of energy, 

 vhich is the expression of "inverse squares." But in the case of a 

 primitive force which is not radiation, (as in gravity, elasticity, cohesion, 

 or affinity,) the law of increment or decrement with distance may have 

 any mathematical value, and may be entirely different and incommen- 

 surable with every variety of force. 



Unfortunately the human mind has been gifted with no insights or 

 intuitions which can determine the ft priori certainty of a single fact of 

 natural law. After twenty-five centuries of vainly-struggling specula- 

 tion, the true law of one kind of force was laboriously ascertained only 

 two little centuries ago. And this result is justly regarded as the most 

 brilliant achievement of the highest human intellect. Did experience 

 teach us that the law of gravity was one of simple decrease of intensity 

 directly with the distance, (in which case the periodic times of the plan- 

 ets would be directly as their distances and their orbital velocities the 

 same at all distances,) or did it teach lis that its energy was precisely 

 the same at all distances, as Faraday thinks to be the true desideratum, 

 (in which case the periodic times as well as the orbital velocities would 

 be as the square roots of the distances,) or did it teach us that its inten- 

 sity increased directly as the distance, as by an elastic bond,* (in which 

 case the periods of revolution would be the same for all distances, and 

 the orbital velocities therefore, proportional to the distance,) in each 

 and every case it would still be unalterably true that the energy ex- 

 pended in separating two bodies would be exactly equal to the energy 

 given out in their return to the antecedent position. And this is what 

 is meant by the " conservation of force." 



Probably no generalization of science has been the occasion of more 

 misapprehension and confusion than this of " conservation.'' Properly 

 speaking, " Force " is not conserved at all ! It is the offspring of Force, or 

 " work " that is really conserved. As words necessarily follow thought 

 practically no less than genetically, (and sometimes longo intervallo,) it 

 results that with the increasing specializations of scientific conception, 

 many words continue to retain their more primitive or "comprehensive 

 type" cf meaning, without originating the required varieties or differ- 

 entiations of expression ; and such has been the case with the very 

 useful word "force;" which is employed sometimes in its more general- 

 ized sense, as including any stress or action whatever; sometimes as 

 limited to quantity of motion ; sometimes as synonymous with energy, 

 (in which sense alone is "conservation" applicable to it;) sometimes as 

 expressing " the mere rate of conversion or transference of energy per 

 unit length of that motion," (with a strong suspicion that " there is 

 probably no such tiling as force at all ; ")t and sometimes as signifying 



* There is reason to believe that this is actually the law of the atomic orbits. 



t Lecture on "Force," by Professor Tait, of Edinburgh. Nature, 2l8t September, 

 1876, vol. xiv, pp. 459, 4G3. It is certain that Newton did not employ the word Vis in 

 any such restricted sense, as the learned professor would im))Iy. 



