works of nature, like that which we perceive among the lower orders of existences. The satellites accompany the planets; the planets follow the sun; and the sun himself is connected with a system of stars, over which presides another sun of superior magnitude and force; and so on, through a number of variations and degrees, which not even the imagination itself

can trace.

The immense distance of these vast bodies, is also another consideration that overpowers all our faculties. James Cassini attempted to show, that the annual parallax of Sirius, which is the nearest of all the fixed stars, is about six seconds; and from this it would follow, that its distance from our earth is near eighteen thousand times greater than that of the sun. But from all the observations hitherto made, it appears, that the parallaxof the stars is altogether insensible, and consequently their distance must be such that no calculation can estimate. If the stars, therefore, be considered as suns, having a number of planets moving round them, like those that compose the solar system, it will be no argument against their existence, to say that we do not see them; for as the suns themselves are at such distances as to appear like so many luminous points, the planets, which shine only by reflection, must be totally invisible.

Dr. Halley has a very ingenious observation upon the magnitudes and distances of the fixed stars. He remarks, that there can be only thirteen points upon

the surface of a sphere, whose distances from each other shall be all equal to the radius; and as the nearest fixed stars are generally considered to be as far from each other as they are from the sun, he supposes them to be placed in the surface of an imaginary concave sphere, which has the sun for its centre; and thence infers, that there can be only thirteen stars of the first magnitude. The stars of the second magnitude he supposes to be twice as far distant from the sun as those of the first, and by placing them in like manner in the surface of a sphere, at such distances from each other as are equal to half the radius, their number will be fifty-two. At a triple distance the surface of a sphere would contain an hundred and seventeen, which, therefore, is the number of stars of the third magnitude; and so on. This supposition is nearly agreeable to the usual method of classing the stars of the first and second magnitude; and if the distribution of them could be made according to their true magnitudes, it would probably be found more accurate.

Halley conceived the whole solar system, together with all the systems of the stars, to be in motion round some point, which is the centre of gravity of the whole; and in pursuing this idea, the following reflections naturally occurred to him: "If," says he, "the number of stars be finite, and occupy only a part of space, it will follow that they must be surrounded by a void. But as this void can have no action upon the bodies which it environs, those bodies must exert all their force

upon one another, without equilibrium, and without compensation. Those which are at the extremities, or near the borders of the void, will be strongly and continually attracted by those near the centre; and these efforts, continued and multiplied through a number of ages, must at length draw all the suns and planets into that point, and form one immense mass, which must for ever remain there, without action and without motion. But if, on the contrary, the number of stars be infinite, and the system without bounds, all the forces will be balanced among themselves; the suns and planets will preserve the paths prescribed them; and the order of the universe will be perpetually the same." These are the conceptions of a vigorous mind; but they lead us into a labyrinth where there is no clue to be found; infinity can be comprehended by God alone.

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LETTER XXI.

OF THE PHENOMENA AND AFFECTIONS OF THE SUN, MOON, AND PLANETS.

THE sun was generally considered by the ancients as a globe of pure fire; but, from a number of dark spots, which, by means of a telescope, may be seen on different parts of his surface, it appears that this opinion was ill-founded. These spots consist, in general, of a nucleus, or central part, which appears much darker than the rest, and seems to be surrounded by a mist or smoke; and they are so changeable in their situation and figure, as frequently to vary during the time of observation. Some of the largest of them, which are found to exceed the bulk of our earth, are often to be seen for three months together; and when they disappear, they are generally converted into faculæ, or luminous spots, which appear much brighter than the rest of the sun. About the time that they were first discovered by Galileo, forty or fifty of them might be frequently seen on the sun at a time; but at present we can seldom observe more than thirty; and there have been periods of seven or eight years, in which none could be seen.

Father Scheiner, who was contemporary with Galileo, has given us an anecdote, which will serve to show how this discovery was at first received. He imagined himself to have been the only person who had ever seen this curious phænomenon, and

having communicated the result of his observations to the provincial of his order, received the following answer. "This subject is not mentioned by any of the ancient philosophers: I have read my Aristotle several times over, and have found nothing like what you speak of. Do not expose yourself by propagating these absurdities; for be assured, that it is only some defect in your eyes or your glasses, which makes you imagine that you see spots in the sun." And so firmly persuaded was this zealous Peripatetic that every thing was contained in the Greek philosophy, that neither the most rational arguments, nor the evidence of his senses, could convince him to the contrary: he continued inflexible in his opinion, and would not suffer Scheiner to publish his discovery, till he had promised him to do it under a fictitious name.

The general opinion concerning the solar spots is, that they are occasioned by the smoke and opaque matter thrown out by volcanos or burning mountains of immense magnitude; and that when the eruption is nearly ended, and the smoke dissipated, the fierce flames are exposed, and appear like faculæ or luminous spots. Lahire imagined the sun to be in a continual state of fusion, and that the spots which we observe are only the eminences of large masses of opaque matter, which, by the irregular agitations of the fluid, sometimes swim upon the surface, and at other times sink and disappear. Others have supposed them to be occasioned by a number of planets, circulating round the sun, at a small distance from his surface.