tems (and whose existence is only hypothetical,) are invisible to us, it is probable, that we also are invisible to them.

Emily. But they may see our sun as we do theirs, in appearance a fixed star?

Mrs. B. No doubt; if the beings who inhabit those planets are endowed with senses similar to ours. By the same rule, we must appear as a moon, to the inhabitants of our moon; but on a larger scale, as the surface of the earth is about thirteen times as large as that of the moon.

Emily. The moon, Mrs. B., appears to move in a different direction, and in a different manner from the stars?

Mrs. B. I shall defer the explanation of the motion of the moon, till our next interview, as it would prolong our present lesson too much.

CONVERSATION VII.

ON THE PLANETS.

OF THE SATELLITES OR MOONS.-GRAVITY DIMINISHES AS THE SQUARE OF THE DISTANCE. OF THE SOLAR SYSTEM.-OF COMETS.-CONSTELLATIONS, SIGNS OF THE ZODIAC.—OF COPERNICUS, NEWTON, &C.

MRS. B.

THE planets are distinguished into primary and secondary. Those which revolve immediately about the sun are called primary. Many of these are attended in their course by smaller planets, which revolve round them these are called secondary planets, satellites, or moons. Such is our moon which accompanies the earth, and is carried with it round the sun.

Emily. How then can you reconcile the motion of the secondary planets to the laws of gravitation; for the sun is much larger than any of the primary planets; and is not the power of gravity proportional to the quantity of matter?

Caroline. Perhaps the sun, though much larger may be less dense than the planets. Fire you know is very light, and it may contain but little matter, though of great magnitude.

Mrs. B. We do not know of what kind of matter the sun is made; but we may be certain, that since it is the general centre of attraction of our system of planets, it must be the body which contains the greatest quantity of matter in that system.

You must recollect, that the force of attraction is not only proportional to the quantity of matter, but to the degree of proximity of the attractive body: this power is weakened by being diffused, and diminishes as the squares of the distances increase. The square is the product of a number multiplied by itself; so that a planet situate at twice the distance at which we are from the sun would gravitate four times less than we do; for the product of two multiplied by itself is four.

Caroline. Then the more distant planets move slower in their orbits; for their projectile force must be proportioned to that of attraction? But I do not see how this accounts for the motion of the secondary round the primary planets, in preference to the sun?

Emily. Is it not because the vicinity of the primary planets renders their attraction stronger than that of the sun?

Mrs. B. Exactly so. But since the attraction between bodies is mutual, the primary planets are also attracted by the satellites, which revolve round them. The moon attracts the earth, as well as the earth the moon; but as the latter is the smaller body, her attraction is proportionally less; therefore neither the earth revolves round the moon, nor the moon round the earth; but they both revolve round a point, which is their common centre of gravity, and which is as much nearer the earth than the moon, as the gravity of the former exceeds that of the latter.

Emily. Yes, I recollect your saying, that if two bodies were fastened together by a wire or bar, their common centre of gravity would be in the middle of the bar, provided the bodies were of equal weight; and if they differed in weight, it would be nearer the larger body. If then the earth and moon had no projectile force which prevented their mutual attraction from bringing them together, they would meet at their common centre of gravity.

Caroline. The earth then has a great variety of mo

tion, it revolves round the sun, upon its axis, and round the point towards which the moon attracts it.

Mrs. B. Just so; and this is the case with every planet which is attended by satellites. The complicated effect of this variety of motions, produces certain irregularities, which, however, it is not necessary to notice at present.

The planets act on the sun in the same manner as they are themselves acted on by their satellites; for attraction, you must remember, is always mutual; but the gravity of the planets (even when taken collectively) is so trifling compared with that of the sun, that they do not cause the latter to move so much as one half of his diameter. The planets do not, therefore, revolve round the centre of the sun, but round a point at a small distance fron its centre, about which the sun also revolves.

Emily. I thought the sun had no motion?

Mrs. B. You were mistaken; for besides that which I have just mentioned, which is indeed very inconsiderable, he revolves on his axis; this motion is ascertained by observing certain spots which disappear, and reappear regularly at stated times.

Caroline. A planet has frequently been pointed out to me in the heavens; but I could not perceive that its motion differed from that of the fixed stars, which only appear to move.

Mrs. B. The great distance of the planets renders their motion apparently so slow, that the eye is not sensible of their progress in their orbit, unless we watch them for some considerable length of time: in different seasons they appear in different parts of the heavens. The most accurate idea I can give you of the situation and motion of the planets, will be by the examination of this diagram, (plate VII. fig. 1.) representing the solar system, in which you will find every planet with its orbit delineated.

Emily. But the orbits here are all circular, and you said that they were elliptical. The planets appear too,