THOUGHTS ON THE NATURE AND ORIGIN OF FORCE. 243 



created. A very striking example of the continual transformation back 

 and forth of these two conditions of force is presented in i)lanetary 

 movements, especially in those of considerable eccentricity. Taking 

 the case of a cometary orbit, for example, whose greatest elongation 

 should extend beyond the distance of Neptune, while its perihelion 

 should lie far within the orbit of Mercury, we should find that the body, 

 rushing or falling toward the sun with accelerated rapidity, would 

 finally acquire a velocity of a hundred or two hundred miles per second; 

 .when its accumulated momentum would suffice to hurl it off as a pro- 

 jectile to its remote aphelion, where its velocity would be reduced to 

 two or three miles per second, and the body be again in a condition to 

 repeat its mighty oscillation. And this majestic pendulum, occupying 

 some three-quarters of a century in its excursion, while exhibiting suc- 

 cessively so enormous an absorption and alternate generation of motion, 

 would at the same time illustrate the constancy and indestructibility of 

 its force, in its transformation from the latent to the actual, and vice 

 versa; the vis-viva of its lowest speed, jAiis its coexisting potential of 

 gravitation at highest elevation, being exactly equal to those of its 

 highest speed and lowest fall. 



And yet, obvious as these truths appear, an acute and compre- 

 hensive thinker in an admirable exposition of the doctrine of evo- 

 lution, has devoted a chapter to the unfortunate fallacy of the 

 " Continuity [or indestructibility] of Motion." If it be once admitted 

 that motion can be transmuted into any other form of force, then of 

 necessity it cannot exist also as motion. The loose brick balanced on 

 the chimney top, ready at a sudden gust of wind to topple and to strike 

 a passer-by to earth, has a potential force, imparted to it perhaps fifty 

 years ago by the hod-carrier and the brick-layer who raised it and 

 placed it in its seat of power. During all these fifty years it has lain 

 there quiet. True, it has partaken of the earth's rotations and revolu- 

 tions; true, it has responded faithfully to all the varying, never-ceasing 

 vibrations of summer-heats and winter-colds; and in all this passive 

 acquiescence it has but illustrated its inertia. But this is manifestly 

 altogether foreign to what we are considering. Whence, then, the motion 

 that, commencing with the fall from the -chimney, hurled back the brick 

 to the earth from which it had so long before been lifted ? The blast 

 that tilted it was but the trigger which set free its latent power. Shall 

 we say that the original motion imparted to it was also latent during 

 all these years 1 Such an expression as " latent motion " is but a sense- 

 less contradiction of terms. The cap-stone of the great pyramid, ele- 

 vated to its dizzy height with much effort and labor, might possibly 

 remain there unmoved forever. 



The simple truth is, that so far from our having any warrant for the 

 assumption that the sum of all the motions in the universe is a constant, 

 the repeated creation and destruction of motion is on the contrary neces- 

 sarily involved in the indestructible transmutability of force. 



