are necessarily ignorant of the length of its day and night, or the variety of seasons it may be liable to. Mercury is 3000 miles in diameter. Large as Mercury, when thus considered, appears to be, it is but an atom when compared with Jupiter, whose diameter is 90,000 miles. Its apparent diameter, at a mean distance from the earth, is 20 seconds.

Mercury is supposed to move at the rate of 110,680 miles per hour. The sun is above 26,000,000 times as big as Mercury; so that it would appear to the inhabitants of Mercury nearly three times larger than it does to us; and its disc, or face, about seven times the size we see it. As the other five planets are above Mercury, their phenomena will be nearly the same to it as to us. Venus and the earth, when in opposition to the sun, will shine with full orbs, and afford a brilliant appearance to the Mercurian spectator.

Mercury, like the moon, changes its phases, according to its several positions with respect to the sun and earth. He never appears quite round or full to us, because his enlightened side is never turned directly towards us, except when he is so near the sun as to become invisible. The times for making the most favourable observations on this planet are, when it when it passes before the sun, and is seen traversing his disc in the form of a black spot: this passage of a planet over the face of the sun is called a transit. It happens in its lower conjunction, at a particular situation of the nodes; which leads us to mention their place in the ecliptic.

C

The angle formed by the inclination of the orbit of Mercury with the plane of the ecliptic, is 16' 59'; the node from which Mercury ascends northward, above the plane of the ecliptic, is 16 1' 30"; in Taurus, the opposite one, 14° 1 24"; in Sagittarius, its nodes move forward about 50" per year.

If Mercury, at his inferior conjunction, comes to either of his nodes about these times, he will appear to transit over the disc of the sun. But in all other parts of his orbit his conjunctions are invisible, because he either above or below the sun.

goes

1

OF VENUS. 오

Venus is the brightest and largest, to appearance, of all the planets, distinguished from them all by a superiority of lustre; her light is of a white colour, and so considerable, that in a dusky place she projects a sensible shade.

The diameter of Venus is 7,699 miles; her distance from the sun is 69,500,000 miles; she goes round the sun in 224 days, 16 hours, 49 minutes, moving at the rate of 80,995 miles per hour. Her motion round her axis has been fixed by some at 23 hours, 22 minutes; by others, at above 24 days. She, like Mercury, constantly attends the sun, never departing from him above 47 or 48 degrees. Like Mercury, she is never seen at midnight, or in opposition to the sun, being visible only for three or four hours in the morning or evening, according as she is before or after the sun.

One would not imagine that this planet, which appears so much superior to Saturn in the heavens, is so inconsiderable when compared to it; for the diameter of Saturn is nearly 78,000 miles; while, on the other hand, one would scarce imagine that Venus, which appears but as a lucid spangle in the heavens, was so large a globe as she truly is; her. diameter being 7,699 miles. It is the distance which produces these effects; which gives and takes away the magnitude of things. Her apparent size varies with her distance; at some seasons she appears near 32 times larger than at others.

When this planet is in that part of its orbit which is west of the sun, that is, from her inferior to her superior conjunction, she rises before him in the morning, and is called phosphorus or lucipher, or the morning star. When she appears east of the sun, that is, fro her superior to her inferior conjunction, she sets in the evening after him; or, in other words, shines in the evening after he sets, and is called hesperus or vesper, or the evening star.

The inhabitants of Venus see the planet Mercury always accompanying the sun; and he is to them, by turns, an evening or a morning star, as Venus is to us. To the same inhabitants, the sun will appear almost twice as large as he does to us.

Venus, when viewed through a telescope, is seldom seen to shine with a full face; but has phases, just like the moon, from the fine thin crescent to the enlightened hemisphere. Her illuminated part is constantly turned towards the sun; hence its

horns are turned towards the east when it is a morning star, and towards the west when it is an evening star. Some astronomers have thought they perceived a satellite moving round Venus; but, as succeeding observers have not been able to verify their observations, they are supposed to have originated in error. In observing the transit of Venus, Mr. Dunn, and other gentlemen, saw a penumbra which took place about five seconds before the contact, preceding the egress of the planet; and from thence they concluded, that it had an atmosphere of about 50 geographical miles in height.

We are told, that when Copernicus first published his account of the solar system, it was objected to him that it could not be true; because, if it was, the inferior planets must have different phases, according to their different situation with respect to the sun and earth; whereas they always appearound to us. The answer said to be made by him is, that they appear round to the eye by reason of their distance; but, if we could have a nearer, or more distinct view of them, we should see in them the same phases as we do in the moon. The invention of telescopes is said to have verified this prediction of Copernicus. But it is neither probable that a defender of the Ptolemaic system should make such an objection, or Copernicus such an answer; since, in the Ptolemaic, as well as in the Copernican system, the shape of these planets ought to change just as the moon dees; consequently, the mere change of shape in the inferior planets is an argument which,

in the common way of urging it, proves nothing at all as to the truth or falsehood of the Copernican system. If, besides the changes of shape made in the inferior planets, we consider the situation of the planets with respect to the sun, when these changes happen, this, indeed, will shew us that the Ptolemaic system is false*; as will be seen in a subsequent part of these Essays.

Venus is sometimes seen passing over the disc of the sun, as a round dark spot. These appearances, which are called transits, happen very seldom; though there have been two within these few years, the one in June 1761, the other in June 1769; the next will be in the year 1874.

OF THE EARTH. Ө

The next planet that comes before us is the Earth that we inhabit: small as it really is, when compared to some of the other planets, it is to us of the highest importance; we wish only to attain knowledge of others, that we may find out their relation to this, and from thence learn our connection with the universe at large. But, when viewed with an eye to eternity, its value to us is heightened in a manner that exceeds expression, and surpasses all the powers of the human mind. He alone can form some idea of it, who, in the regions of celestial bliss,

*Rutherford's System of Natural Philosophy, vol. ii. p. 781.