THE RHYTHM OF MOTION. 281 



forces — a postulate which, as we have seen, is necessitated 

 by the form of our experience — and rhythm is an inevitable 

 corollary from the persistence of force. 



[Note. — In the Edinburgh Review for January, 1884, there was an antag- 

 onistic criticism of this work. The writer of the criticism, Lord Grimthorpe, 

 made much of the exception furnished by non-periodic comets to the law 

 above set forth. I was about to admit this exception when, on looking into 

 the matter, I found no need for doing so. Though five or six cometary orbits 

 are said to be hyperbolic, yet, as I learn from one who has paid special atten- 

 tion to comets (having tabulated the directions of their aphelia), 4< no such 

 orbit has, I believe, been computed for a well-observed comet." Hence the 

 probability that all the orbits are ellipses is overwhelming. Ellipses and 

 hyperbolas have countless varieties of forms, but there is only one form of 

 parabola ; or, to speak literally, all parabolas are similar, while there are in- 

 finitely numerous dissimilar ellipses and dissimilar hyperbolas. Consequently, 

 anything coming to the Sun from a great distance must have one exact amount 

 of proper motion to produce a parabola : all other amounts would give hyper- 

 bolas or ellipses. And if there are no hyperbolic orbits, then it is infinity to 

 one that all the orbits are elliptical.] 



20 



