mon burning glass; the action of which is nothing more than this:—all the rays, both of light and heat, which fall on the surface of a lens, are brought into one narrow bundle, by what is called the refracting power of the glass. If, for instance, the glass be one inch in diameter, and we find the focus, which is a little bright spot or image of the Sun formed at a certain distance behind the glass, to be one-tenth of an inch in diameter, then will the light and heat be concentrated one hundred times, that is, the effects of a certain quantity of light and heat are assembled into a space only one hundredth part of the former space:-from this statement, however, some slight deduction will have to be made, on account of some of the rays being reflected from, and others absorbed by, the glass.

The substance of the Sun is considered to be of a weight and density corresponding to that of water; but, nevertheless, the extent of its bulk is such, that if the weight and density of the whole Earth be regarded as 1, the Sun's weight and density may be set down as 333,000.

Such are the general characteristics of the solar globe. Many other effects, produced by its means, we shall treat of in the proper place. It may be sufficient now to observe, that the action of the Sun's light and heat is the direct source of most of the processes which go on upon the Earth, whether relating to the atmosphere or to the vegetable and animal kingdoms.

CHAPTER V.

MERCURY. THE TERM PLANET DEFINED. PRIMARY AND SECONDARY, &c. MOTIONS OF. LUMINOSITY. PHASES. TRANSITS. CONJUNCTION AND OPPOSITION OF PLANETS. -VENUS. MOTIONS OF. PHASES. MORNING AND EVENPARALLAX.

ING STAR.

MERCURY.

We now proceed to the consideration of the bodies which revolve round the Sun, and shall treat of them in the order of their distances, the nearest first.

We should remark, before proceeding further, that the word planet signifies a wanderer; an appellation given to those heavenly bodies, which seemed to move in circles in the heavens. Hence this distinction may refer both to the planets usually so called, and to their moons; the former of which are styled primary, the latter secondary, planets. Again, the primary planets are distinguished into interior or inferior; and exterior or superior:—the first pair of terms pertaining to the two planets whose orbits are between the Sun and the orbit of the Earth; the second pair referring to the planets more distant from the Sun than the Earth.

The planet nearest to the Sun, as far as we know for certain, is one,

whose disk

Can scarce be caught by philosophic eye,

Lost in the near effulgence of his blaze.

This is Mercury, which is somewhat more than thirtysix millions of miles distant from the Sun, and is

about three thousand one hundred and twenty-three miles in diameter. He revolves on his axis in about twenty-four hours five and a half minutes, being a little more than the time occupied by the Earth in revolving on her axis. He moves round the Sun in an elliptical orbit, (as we said do all the other planets,) and when we speak of the distance between him and the Sun, it must be understood that we refer to the mean distance. The time which he takes to travel round the Sun is almost eighty-eight days; a period which constitutes his year: for we must bear in mind, as will be hereafter more particularly shown, that our year is nothing more than the length of time which the Earth occupies in going once round the Sun. Hence, when we speak of a year as connected with the motion of Mercury, we must remember that his year is different from ours,the latter being rather more than four times as long as the former.

As he revolves round his orbit of seventy-two millions of miles in diameter in about eighty-eight days, he has a velocity of motion of about one hundred thousand miles an hour; a rate of which we can form some conception by considering that one hundred thousand miles is about four times the circumference of the earth.

As Mercury is so much nearer the Sun than the Earth, it follows that the amount of light and heat received by this planet is much greater than that received by the Earth; and it has been estimated that it is about seven times greater. Another effect of the comparative proximity of Mercury is, that he never appears so far removed from the Sun as the other planets. At no time is he more than 30° distant from the Sun; that is, if at any time we draw a line

from the Earth to the Sun, and another from the Earth to Mercury, those two lines will never form a larger angle than 30°. It is, in part, for this reason that we do not usually speak of Mercury as a morning or an evening star; he never rises much before the Sun, nor does he set much after the Sun: and, in the next place, owing to the proximity of the Sun, by means of which the rays of this planet are, as it were, drowned, we seldom see him at any time. Not but that, in very clear weather, this planet may be seen just before sunrise in the morning, or just after sun-set in the evening, when it appears a good way off from the Sun. His appearance is brighter than that of Venus, and has a light blue tint about it. He subtends an angle of from 5" to 12" of a degree, according to his position in respect of the Earth.

There is a circumstance which satisfactorily proves that Mercury does not shine by virtue of any light of his own, but merely by light reflected from the Sun. This circumstance, which also applies to some of the other planets, is the existence of phases*, similar to those which are periodically observed in our moon. When Mercury appears at his greatest distance or elongation from the Sun, his illuminated surface has nearly the form of a half circle, more or less, according to the position of the Earth. But when he is passing round on the opposite side of the Sun to that at which the Earth is situated, the illuminated portion becomes more than a semicircle, and assumes that form which is called gibbous; which phase may be represented by placing a semicircle and a semioval with their flat

*This is a word from the Greek, and signifies appearances.

edges in contact. When Mercury is at his greatest distance from the Earth, the Sun is then between them, and shuts out the view of Mercury altogether, so that the whole of the illuminated disk of Mercury can never be seen from the Earth; but when he emerges on the other side of the Sun, he becomes again visible, and his illuminated surface approaches more and more to the form of a semicircle, as he travels on to the position in which he forms a right angle with the Sun and the Earth. As he proceeds in his orbit, he approaches nearer to the Earth, and his semicircular phase becomes diminished to a crescent, which attains its smallest dimensions, when Mercury is either exactly over, or exactly under the Sun; in which position only a few of the Sun's rays, reflected from the surface of Mercury, can reach the Earth, and even those few are rendered almost imperceptible by the superior flood of light, which proceeds directly from the Sun to the Earth. These observations apply likewise to the planet Venus.

It is by such evidence as this that we know that a planet does not appear luminous by any inherent light of its own, but that we see it, in consequence of its reflecting the solar light to the Earth. There is every reason to believe that the Earth reflects light from its surface, just as we have described with respect to Mercury; and that if there be inhabitants in any of the other planets, they see the Earth under the various forms of crescent, semicircular, and gibbous. To this conclusion we arrive, both by analogy, and by certain appearances of the Moon during a solar eclipse, to which we shall hereafter refer more particularly.

A line drawn from the Earth to the Sun is said to