Great difficulty has been experienced in accounting for the origin of comets upon the nebular hypothesis. They approach the sun from all directions, and their motions, in relation to the planets, are as often retrograde as direct. Not only are their orbits excessively elliptical, but they are also inclined to the ecliptic at all angles from 0° to 90°. It is evidently impossible to account satisfactorily for the origin of comets if we assume them all to have been evolved out of the solar nebula, although this has been attempted by M. Faye and others. Comets are evidently, as Laplace and Professor A. Winchell both conclude, strangers to our system, and have come from distant regions of space. If they belonged to the solar system they could not, says Professor Winchell, have parabolic and hyperbolic paths. “Only a small portion of the comets,” he remarks, “are known to move in elliptic orbits.”[8] This assumption that they are foreigners will account for all the peculiarities of their motions; but how are we to account for their coming into our system? How did they manage to leave that system in which they had their origin? If a comet have come from one of the fixed stars trillions of miles distant, the motion by which it traversed the intervenient space could not, possibly, have been derived from gravity. We are therefore obliged to assume that the motion was a projected motion. Comets, in all probability, have the same origin as meteorites. The materials composing them, like those of the meteorites, were probably projected from nebulæ by the expulsive force of the heat of concussion which produced the nebulæ. Some of them, especially those with elliptic orbits, may have possibly been projected from the solar nebula. [Footnote 8: _World Life._ p. 27.] V. _Nebulæ._ It is a curious circumstance that the theory here advanced seems to afford a rational explanation of almost every peculiarity of nebulæ, as I have, on former occasions, endeavoured, at some length, to prove.[9] [Footnote 9: _Philosophical Magazine_, July 1878; _Climate and Cosmology_, Chap. xix.] 1. _Origin of nebulæ._—We have already seen that the theory affords a rational account of the origin of nebulæ.