which produces the variation in the case of Delta Cephei, whose period is only 5 days, 8 hours, is the same which produces all the variety of change which happens in the star Gamma in the Swan's breast, whose periodical changes are completed only in eighteen years. It is not unlikely that a rotation round an axis, which has the effect of presenting different sides of the star of more or less degrees of obscurity or brightness to the eye of a spectator, will account for the phenomena of such stars as Eta Antinoi and Delta Cephei; but it does not appear probable that a motion of rotation is so slow in any of these bodies as to occupy a period of eighteen years, as in the case of the star in the breast of the Swan.

I am disposed to consider it as highly probable, that the interposition of the opaque bodies of large planets revolving around such stars may, in some cases, account for the phenomena. It is true that the planets connected with the solar system are so small in comparison of the sun, that their interposition between that orb and a spectator at an immense distance would produce no sensible effect. But we have no reason to conclude that in all other systems the planets are formed in the same proportion to their central orbs as ours; but, from the variety we perceive in every part of nature both in heaven and earth, we have reason to conclude that every system of the universe is in some respects different from another. There is no improbability in admitting that the planets which revolve round some of the stars may be so large as to bear a considerable proportion (perhaps one half or one third) to the diameters of the orbs around which they revolve; in which case, if the plane of their orbits lie nearly in the line of our vision, they would, in certain parts of their revolutions, interpose between our eye and the stars, so as to hide for a time a portion of their surfaces from our view while in that part of their orbits which is next the earth. Such a supposition is by no means inconsistent with the operation of the law of universal gravitation; for although such planets bore a considerable proportion to the size of their central luminaries, yet we have only to suppose that their density is very small. They may be globes whose central parts are devoid of solid matter, consisting only of a solid external shell for the support of inhabitants, as is probably the case with the planet Saturn, whose density is only equal to that of cork.

cession to pass between our eye and the star to account for the appearance they present; a supposition which perfectly agrees with the idea of a system of revolving bodies.

As it is not probable that the changes of all such stars arise from the same cause, what should hinder us from supposing that there are stars or suns that revolve around planets of a size immensely greater; the planets, for example, bearing a similar proportion to the stars as the sun bears to Jupiter? Considering the immense variety of celestial mechanism throughout the universe, there can be no great improbability in such a supposition. The case of double stars demonstrates that one sun actually revolves round another; and why may not a sun revolve around a central planet, whose surface may contain forty times the area of all the planets of our system, in order to distribute light and heat, and other beneficial influences, to its numerous population? No violation of the law of universal gravitation is implied in such a supposition; and the Almighty is not confined to one mode of arranging systems and worlds. Supposing, then, such an arrangement to exist, it might account for the phenomena of some of the variable stars, particularly those which remain invisible for a certain period. Such are some of those formerly noticed, as the star in Hydra, and that in the breast of the Swan, and particularly a star in the Northern Crown, whose right ascen, is 15h 40', north declin. 28° 49', and period 10 months, and which decreases from the sixth to the ninth and tenth magnitude. It attained its full brightness about the 11th of August, 1795, and continued so for three weeks; in three and a half weeks it decreased to the tenth magnitude, and a few days afterward disappeared. After being a considerable time invisible, in April, 1796, it again appeared; on the 7th of May it reached the ninth magnitude, and then gradually attained its full brightness. If, then, such a star was revolving round a very large central planet, it is easy to conceive that in the more distant part of its course it might be hidden from our view, either in whole or in part, by the interposition of the opaque central body, as is obvious from an inspection of figure 12. And as the star now alluded to never exceeds in lustre a star of the sixth magnitude, it is not improbable that it is one of the inferior order of those luminous orbs which may revolve round an opaque body of superior magnitude.