The stars appear of a sensible magnitude to the naked eye, because the retina is not only affected by the rays of light which are emitted directly from them, but by many thousand more, which, falling upon our eye-lids, and upon the visible aerial particles about us, are reflected into our eyes fo ftrongly, as to excite vibrations, not only in those points of the retina, where the real images of the stars are formed, but also in other parts round about it. This makes us imagine the stars to be much bigger, than they would be if we saw them only by the few rays which come directly from them to our eyes, without being intermixed with others. Any one may be made fenfible of this, by looking at a star of the first magnitude, through a long narrow tube; which, though it takes in as much of the fky as would hold a thousand of such stars, scarce renders that one visible.

The number of the stars almost infinitely exceeds what we have yet been speaking of. An ordinary telescope will discover, in several parts of the heavens, ten times as many stars as are visible to the naked eye. Hooke, in his Micrographia, fays, that with a telescope of twelve feet he dif covered seventy-eight ftars among the pleiades, and with a more perfect telescope, many more.

Galileo

ment.

Galileo reckoned eighty in the space between the belt and the fword of Orion, and above five hundred more in another part of the fame conftellation, within the compass of one or two degrees fquare. Antonia Maria de Rheita counted in the fame conftellation above two thoufand ftars. Future improvements in telescopes may enable us to discover numberlefs ftars that are now invifible; and many more there may be, which are too remote to be feen through telescopes, even when they have received their ultimate improveDr. Herschel, to whose ingenuity and affiduity the aftronomical world is fo much indebted, and of whose discoveries we shall speak more largely in another part of this effay, has evinced what great discoveries may be made by improvements in the inftruments of observation. In fpeaking here of his difcoveries, I fhall ufe the words of M. de la Lande.* "In paffing rapidly over the heavens with his new telescope, the univerfe increased under his eye; 44000 ftars, seen in the space of a few degrees, feemed to indicate that there were feventy-five millions in the heavens." But what are all thefe, when compared to thofe that fill the whole expanse, the boundlefs

Memoires de l'Academie de Dyon, 1785.

boundless fields of ether; indeed, the immensity of the universe must contain fuch numbers, as would exceed the utmoft ftretch of the human imagination. For who can say, how far the universe extends, or where are the limits of it? where the Creator stayed "his rapid wheels;" or where he "fixed the golden compaffes?"

OF THE PLANETS, AS SEEN FROM THE SUN.

Our folar obferver having attained a competent knowledge of the fixed ftars, will now apply himfelf to confider the planets: these, as we have already observed, he will soon distinguish, by their motion, from the fixed ftars; the stars always remaining in their places, but the planets would be feen paffing by them with unequal velocities. Thus on observing the earth, for instance, he will find it moving among the fixed ftars, and approaching nearer and nearer to the more castern ones; in a year's time it will complete it's revolution, and return to the fame place again.

He will find seven of these bodies which revolve round the fun, to each of which he will affign a name, calling the fwifteft MERCURY, denominating the others in order, according to their velocities, as VENUS, then the EARTH, and

afterwards MARS, JUPITER, SATURN, and the GEORGIUM SIDUS,

Proceeding with attention in thus exploring and examining the heavens, he will perceiver that the earth is always accompanied by a fmall ftar, Jupiter by four, Saturn by five, and the Georgium Sidus by two: thefe fometimes precede, at others follow; now pass before, and then behind the planets they respectively attend. These small bodies he will call SECONDARY PLANETS, SATELLITES, or MOONS.

The obferver by remarking the exact time when each planet paffes over fome fixed star, and the time they employ from their fetting out, to their return to the same star again, will find the times elapfing between each fucceffive return of the fame planet to the fame ftar, to be equal; and he would fay, that the feveral planets: describe circles in different periods; but that cach of them always completes it's own circle in the fame fpace of time.

He will further obferve, that there are certain bodies, which at their first appearance are small, obfcure, ill-defined, and that move very flow, but which afterwards increase in magnitude,

5

light,

light, and velocity, until they arrive at a certain fize, when they lofe thefe properties, and diminish in the fame manner as they before aug mented, and at laft disappear. To these bodies, which he will find in all the regions of the heavens, moving in different directions, he will give the name of COMETS.

OF THE PATHS OF THE PLANETS.

Our obferver will take notice, that the planets run fucceffively through thofe conftellations which he has denominated, Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricornus, Aquarius, Pisces; and that they never move out of a certain space, or zone, of the heavens, which he will call the

ZODIAC.

He will find, by proceeding in his obfervation, that the orbits of the planets are not all in the fame plane, but that they crofs each other in different parts of the heavens; fo that if he makes the orbit of any planet a standard, and confiders it as having no obliquity, he would judge the paths of all the reft to be inclined to it; each planet having one half of it's path on one fide, and the other half on the oppofite fide of