64 THE POPULAR SCIENCE MONTHLY,
as for that matter, in the last analysis, all forces do. If there really be an *' ethex-," 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 Schiapai-elli, 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 fi-om about eleven miles a second to five. Now, it is very diflicult, 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