192 THE CONTINUITY OF MOTION. 



continuity of Motion, as well as the indestructibility of 

 Matter, is really known to us in terms of Force. 



§ 59. And now we reach the essential truth to be here 

 especially noted. All proofs of the Continuity of Motion in- 

 volve the postulate that the quantity of force is constant. Ob- 

 serve what results when we analyze the reasonings by which 

 the Continuity of Motion, as here understood, is shown. 



A particular planet can be identified only by its con- 

 stant power to affect our visual organs in a special way. 

 Further, such planet has not been seen to move by the astro- 

 nomical observer; but its motion is inferred from a com- 

 parison of its present position with the position it before 

 occupied. If rigorously examined, this comparison proves 

 to be a comparison between the different impressions pro- 

 duced on him by the different adjustments of his observing 

 instruments. And, manifestly, the validity of all the in- 

 ferences drawn from these likenesses and unlikenesses, 

 depends on the truth of the assumption that these masses 

 of matter, celestial and terrestrial, will continue to affect 

 his senses in exactly the same ways under the same con- 

 ditions; and that no changes in their powers of affecting 

 him can have arisen without force having been expended 

 in working those changes. Going a step further 



back, it turns out that difference in the adjustment of his 

 observing instrument, and by implication in the planet, is 

 meaningless until shown to correspond with a certain calcu- 

 lated position which the planet must occupy, supposing that 

 no motion has been lost. And if, finally, we examine the 

 implied calculation, we find that it takes into account those 

 accelerations and retardations which ellipticity of the orbit 

 involves, as well as those variations of velocity caused by 

 adjacent planets — we find, that is, that the motion is con- 

 cluded to be indestructible not from the uniform velocity 

 of the planet, but from the constant quantity of motion 

 exhibited when allowance is made for the motion communi- 



