64 THE POPULAR SCIENCE MONTHLY. 



as for that matter, in the last analysis, all forces do. If there really 

 be an " ether," then it would seem that somehow all attractions and 

 repulsions of ponderable matter must be due to its action. Challis's 

 investigations and conclusions as to the effect of hydrodynamic actions 

 in such a medium do not seem to have commanded general acceptance ; 

 and the field still lies open for one who will show how gravitation 

 and other forces can be correlated with each other through the ether. 



Meteors and the comets, seeming to belong neither to the solar 

 system nor to the stellar universe, present a crowd of problems as diffi- 

 cult as they are interesting. Much has undoubtedly been gained dur- 

 ing the last few decades, but in some respects that which has been 

 learned has only deepened the mystery. The problem of the origin 

 of comets has been supposed to be solved to a certain extent by the 

 researches of Schiaparelli, Heis, Professor Newton, and others, who 

 consider them to be strangers coming in from outer space, sometimes 

 "captured" by planets, and forced into elliptic orbits, so as to become 

 periodic in their motion. Certainly this theory has strong supports 

 and great authority, and probably it meets the conditions better than 

 any other yet proposed. But the objections are really great, if not 

 insuperable — the fact that we have so few, if any, comets moving in 

 hyperbolic orbits, as comets met by the sun would be expected to 

 move ; that there seems to be so little relation between the direction 

 of the major axes of cometary orbits and the direction of the solar 

 motion in space ; and especially the fact, pointed out and insisted 

 upon by Mr. Proctor in a recent article, that the alteration of a comet's 

 natural parabolic orbit to the observed elliptic one, by planetary action, 

 implies a reduction of the comet's velocity greater than can be reason- 

 ably explained. If, for instance, Brorsen's comet (which has a mean 

 distance from the sun a little more than three times that of the earth) 

 was really once a parabolic comet, and was diverted into its present 

 path by the attraction of Jupiter, as generally admitted, it must have 

 had its velocity reduced from about eleven miles a second to five. 

 Now, it is very difficult, if not out of the question, to imagine any 

 possible configuration of the two bodies and their orbits which could 

 result in so great a change. While I am by no means prepared to 

 indorse as conclusive all the reasoning in the article referred to, and 

 should be very far from ready to accept the author's alternative theory 

 (that the periodic comets have been ejected from the planets, and so 

 are not their captives, but their children), I still feel that the difficulty 

 urged against the received theory is very real, and not to be evaded, 

 though it may possibly be overcome by future research. 



Still more problematical is the constitution of these strange objects 

 of such enormous volume and inconceivable tenuity, self-luminous and 

 transparent, yet reflecting light, the seat of forces and phenomena 

 unparalleled in all our other experience. Hardly a topic relating to 

 their appearance and behavior can be named which does not contain 



