The equatorial diameter of Jupiter is more than 6,000 miles longer than his polar one. Hence, when viewed through a good telescope, he appears to be oval.

DENSITIES OF THE PLANETS.-The density of bodies is ascertained by comparing their bulk with their weight-or, in other words, with the quantity of matter which they contain. And as the power of attraction in any body is in proportion to the quantity of matter which it contains, the densities of the planets have been determined by comparing their magnitudes with the power of attraction which they exert on other bodies in similar circumstances.

According to the most recent authorities, if the density of water be taken as 1, the sun will be 144; of Mercury, 6-85; of Venus, 4-81; of the Earth, 5.66; of the Moon, 35; of Mars, 401; of Jupiter, 137; of Saturn, 0.75; of Uranus, 1.28.

The density of the sun is little more than the density of water, while the average density of the earth is more than five times that of water. Hence the density of the earth is nearly four times as great as the density of the sun; but so great is the magnitude of the sun, that it contains about 333,000 times as much matter as the earth— or more than 600 times as much as all the planets taken together!

THE SUN'S ATTRACTION, LIGHT, HEAT, &c.—The sun's attraction, light, heat, and apparent magnitude with respect to the planets, are supposed to be inversely proportional to the squares of their distances from him. Hence, if the earth were where Mercury is, the sun would appear to us nearly seven times as large as it now does, and his heat, light, and attractive powers would be increased in the same proportion; for the squares of the distances of the earth and Mercury from the sun are nearly as 7 to 1. Again, Urănus's distance from the sun, compared to Saturn's, is as 2 to 1; and, consequently the attraction, heat, light, and apparent magnitude of the sun at Uranus are four times less (the square of 2) than they are at Saturn. We are not, however, to measure the degrees of heat and cold experienced at the planets with reference to our own. We are ignorant of their structure, surfaces, and atmospheres, and we cannot therefore know what effect may be produced upon them by the solar rays. We have no reason, therefore, to conclude that the planets are either too warm or too cold to be inhabited. One thing we may conclude― that the All-wise and All-good Creator has made nothing in vain.

THE MOON.-The Moon revolves round the earth in a month, and with the earth she is carried round the sun in the course of a year. And as she always presents the same face or side to the earth, it follows that she must turn once round her axis in the course of a month. For if she had no rotation on her axis, every part of her surface would be presented to the earth in the course of her revolution round it. Hence, as the moon turns but once round her axis in a month, before the sun, her day and night must be each nearly a fortnight long. And as the moon enlightens the earth by reflecting the light

of the sun, so the earth illumines that side of the moon that is next to it, while turned away from the sun. The half of the moon which is towards the earth may be said, therefore, to have no darkness at all; for during the fortnight in which it is turned away from the sun, the earth shines upon it with a disc sixteen times as large as that of the full moon. The other half of the moon receives no light from the earth at all, and is therefore in darkness for the half of every month, with the exception of the light which it receives from the stars.

The distance of the moon from the earth is about 240,000 miles. Her apparent diameter is about equal to that of the sun's, though it is really 400 times less; but this is because she is 400 times nearer to us than the sun is.

When the moon is between the earth and the sun, her enlightened hemisphere is turned from us, and the side which is next to us is darkened, and therefore invisible. She is then said to change. As she proceeds in her course she turns a bright edge towards us, which we call the new moon. If we observe her next evening, we shall find that she has moved about 13° farther east of the sun than on the preceding evening, and that her crescent of light has increased in breadth. Repeating our observations, we shall find that, as she progresses eastward from the sun, her enlightened surface comes more and more into view till she arrives at her first quarter, and comes to the meridian at sunset. She has then completed half her course, from the new to the full moon; and as half of her enlightened dise is turned towards the earth, we say it is half moon. After her first quarter she is said to be gibbous,a because she presents more than half of her enlightened hemisphere to the earth. And as she recedes farther and farther from the sun, she appears more and more gibbous, till she completes half of her revolution round the earth, and is seen rising in the east when the sun is setting in the west. She then presents the whole of her enlightened hemisphere to the earth, and it is then said to be full moon. In this position the moon is said to be in opposition, because she is then on the opposite side of the earth with respect to the sun; or, in other words, the earth is between her and the sun. And at new moon she is said to be in conjunction,

because she is between the earth and the sun.

As she proceeds in her orbit she becomes gibbous again, and presents the same changes as before, but in an inverted order, till we see her in the morning like a fine thread of light, a little to the west of the rising sun. For the next day or two she rises in conjunction with the sun, and is consequently lost to our view till, having passed the sun to the eastward, we hail her appearance again as the NEW MOON!

From gibbus, a Latin word for hunch-backed, convex.

The crescent or illuminated part of the moon is always turned in the direction of the sun. After her conjunction, therefore, or while she is increasing, the convex part of the crescent is turned to the west, and the horns or hollow part of it to the east. Before her conjunction, or while she is waning, the reverse takes place. These different appearances of the moon are called her PHASES, and they prove that she shines not by her own light; for if she did, she would always present to us a full enlightened orb like the sun.

If we observe the motion of the moon in connexion with the position of any fixed star, we shall be convinced that she moves from west to east, and not from east to west, as she appears to do. For if the star is to the eastward of the moon, the distance between them will gradually diminish till they appear in the same direction from our eye. The moon will then pass to the eastward of the star, and the distance between them will gradually increase. In 24 hours after we shall find that the moon has moved 13 degrees to the eastward; and if we continue our observations we shall find that in 27 days, 7 hours, 43 minutes, and 4 seconds, she having made the circuit of the heavens, will again be in a line with the same fixed star. This is called a periodic or sidereal revolution of the moon. But though the moon makes a complete revolution round the earth in 27 days, 7 hours, 43 minutes, and 4 seconds, it requires 2 days and 5 hours additional to bring her to that position in which the same face will be presented to the sun. This period, which is called a synodical month, consists of 29 days, 12 hours, and 44 minutes, and it is reckoned from new moon to new moon. This difference arises from the earth's annual motion in her orbit; for while the moon is revolving round the earth, the earth is advancing in her orbit. The moon, therefore, after completing one revolution, will have to move several degrees farther before she can come again into the same position with respect to the earth and sun. This may be illustrated by the motions of the hands of a watch. At 12 o'clock they start together, and at one, the minute-hand, having made a complete revolution round the dial, is on a line with the figure XII. But, in the meantime, the hour-hand has moved forward in its course as far as the figure I; and it will consequently take the minute-hand somewhat more than five minutes, in addition to the hour, to overtake it.

MAGNITUDE OF THE EARTH.-As the magnitude of the earth is the scale or standard by which we are enabled to form a conception of the magnitudes of the heavenly bodies, and of the immensity of the uni verse, we should endeavour to impress the minds of our pupils with adequate ideas of its vast extent. In a preceding part of this book its dimensions as a globe, and the extent of its surface in square miles have been given (pp. 11 and 45); but, as mere calculations are seldom realized by young persons, something should be done to give them a practical and intuitive proof of its amazing magnitude. With this view they should be conducted to some elevated place in the neighbourhood,

and be told to look around them; and however extensive the landscape before them may be, they should be told that it is little more than a mere spot, when compared with the whole extent of the surface of the earth! For even if it should comprise a circle of 150 miles in circumference, it would scarcely amount to the hundred-thousandth part of the earth's surface. We should therefore have to conceive 100,000 landscapes as large as the one we are contemplating, before we could form an adequate idea of the magnitude of the earth!

MAGNITUDE OF THE SUN.-But what is the magnitude of the earth, amazing as it is, when compared to the magnitude of the sun? The length of the sun's diameter is, as we have seen, about 860,000 miles that is, nearly four times the distance of the moon from the earth (240,000). Hence, if the centre of the sun were in the exact situation in which the centre of the earth now is, its surface or body would extend to the moon, and 200,000 miles beyond it; or, in other words, about twice as far as the moon!

Again, if the sur were a hollow sphere, and our earth, as large as it now is, in the centre of it, the distance between the earth and the inner surface of the sun would be 440,000 miles. Half-way between us and the inner or concave surface of the sun, might be the moon, as large as she now is, and at the same distance from us; and if perforations were made in the surface of the sun, so as to admit the luminous matter with which it is covered, to represent the stars, the appearance presented to us would differ little from that of the visible heavens; that is, the concave surface of the sun would appear to be as distant and as large as the whole universe appears to the ordinary observer.

ZODIACAL LIGHT.-The nature of this beautiful and interesting phenomenon is not yet known, though it was noticed so far back as 1683 by the Elder Cassini. Some astronomers suppose it to be the denser portion of an ether diffused through space, and sufficiently massive beyond the orbit of Venus to reflect light. Humboldt, in his "Cosmos," considers it "a vapoury flattened ring freely revolving in space between the orbits of Mars and Venus." Its appearance is that of a conical-shaped light extending from the horizon nearly along the course of the ecliptic, the vertex attaining distances of 70° or 80° from the sun's place. In these latitudes it is visible in spring before sunset, and in autumn after sunset. It is much more brilliant in tropical climates.

ASTRONOMICAL TERMS.

[As definitions or explanations of the following TERMS have been given in the preceding part of this work, it will be sufficient to refer the reader to the pages in which each may be found.]

SUPPLEMENTARY DEFINITIONS.-An Artificial Globe is a miniature representation of the earth or heavens. The one that represents the earth is called the Terrestrial, and the one that represents the heavens the Celestial Globe. Eich globe is hung in a brass ring called the Brazen or Universal Meridian, and turns upon an axis or wire, which passes through each pole. The Brazen Meridian is divided into four quadrants of 90 degrees each, two of which begin at the equator and increase towards the poles, which serve to show the latitude of places on the Terrestrial Globe, and the declination of the sun, moon, and stars, on the Celestial. The other two quadrants are numbered from the poles to the equator, and serve to elevate or depress the poles above or below the horizon for any latitude.