greatest on the other; sometimes in a straight line, at others in an elliptic curve.

When a satellite is in its superior semicircle, or that half of its orbit that is more distant from the earth, its motion appears direct to us; when a satellite is in its inferior semicircle, nearest to the earth, the apparent motion of it is retrograde. Both these motions seem quickest, when the satellite is nearest the centre of the primary, and slower when they are more distant; at the greatest distance they appear stationary, for a short time.

The satellites, and their primaries, mutually eclipse each other, in the same manner in which it has been shewn that the earth and the moon do. But there are three cases in which the satellites disappear

to us.

The one is, when the satellite is directly beyond the body of its primary, with respect to the earth ; this is called an occultation of the planet.

Another is, when it is directly behind its primary, with respect to the sun, and so falls into its shadow, and suffers an eclipse, as the moon, when the earth is interposed between that and the sun.

The last is, when it is interposed between the earth and its primary; for then it cannot be distinguished from the primary itself.

It is not often that a satellite can be discovered upon the disc of Jupiter, even by the best telescopes; excepting at its first entrance, when, by reason of its being more directly illuminated by the rays of the

sun, than the planet itself, it appears like a lucid spot upon it; sometimes, however, a satellite is seen passing over the disc like a dark spot; this has been attributed to spots on the surface of the satellite; and, that the more probably, as the satellitehas been known to pass over the disc, at one time as a dark spot, and at another time to be so luminous, as only to be distinguished from the planet at its ingress and egress. The beginning and endings of these eclipses are easily seen by a telescope, when the planet is in a proper situation; but when it is in conjunction with the sun, the brightness of that luminary renders both the planet and satellite invisible.

By observing the eclipses of Jupiter's satellites, it was discovered, that light is not propagated instantaneously, though it moves with an incredible velocity; so that light reaches from the sun to us in the space of eleven minutes of time, at more than the rate of 100,000 miles in a second.

The orbits of all the satellites of Saturn, except the fifth, are nearly in the same plane, which plane makes an angle with that of Saturn's orbit, of about 31°; this inclination is so great, that they cannot pass either across Saturn or behind it, with respect to the earth, except when they are very near their nodes; so that their eclipses are not near so frequent as those of Jupiter. An occultation of the fourth, behind the body of Saturn, has been observed; and Cassini once saw a star covered by the fourth satellite, so that for 13 minutes they appeared as one.

Ι

OF ECLIPSES.

Those phenomena, that are termed eclipses, were, in former ages, beheld with terror and amazement, and looked upon as prodigies that portended calamity and misery to mankind. These fears, and the erronious opinions which produced them, had their source in the hieroglyphical language of the first inhabitants of the earth. We do not, however, imagine that even the most ancient of these knew any more of the laws and motions of the heavenly bodies, than what could be discovered from immediate sight; or, that they knew enough of the lunar system to calculate an eclipse, or even that they ever attempted it.

The word eclipse is derived from the Greek, and signifies dereliction, a fainting away or swooning. Now, as the moon falls into the shadow of the earth, and is deprived of the sun's enlivening rays, at the time of her greatest brightness, and even appears pale and languid before her obscuration; lunar eclipses were called lunæ labores, the struggles or labours of the moon; to relieve her from these imagined distresses, superstition adopted methods as impotent as they were absurd.

When the moon, by passing between us and the sun, deprived the earth of its light and heat, the sun was thought to turn away his face, as if in abhorrence of the crimes of mankind, and to threaten everlasting

night and destruction to the world. But, thanks to the advancement of science, which, while it has delivered us from the foolish fears and idle apprehensions of the ancients, leaves us in possession of their representative knowledge, enables us to explain the appearances on which it was founded, and points out the perversion and abuse of it.

Any opake body, that is exposed to the light of the sun, will cast a shadow behind it. This shadow is a space deprived of light, into which, if another body comes, it cannot be seen for want of light; the body thus falling within the shadow, is said to be eclipsed.

The earth and moon being opake bodies, and deriving their light from the sun, do each of them cast a shadow behind, or towards the hemisphere opposed to the sun. Now, when either the moon or the earth passes through the other's shadow, it is thereby deprived of illumination from the sun, and becomes invisible to a spectator on the body from whence the shadow comes; and such spectator will observe an eclipse of the body which is passing through the shadow; while a spectator on the body which passes through the shadow, will observe an eclipse of the sun, being deprived of his light.

Hence there must be three bodies concerned in an eclipse; 1. the luminous body; 2. the opake body that casts the shadow; and, 3. the body involved in the shadow.

OF ECLIPSES OF THE MOON.

As the earth is an opake body, enlightened by the sun, it will cast a shadow towards those parts that are opposite to the sun; and the axis of this shadow will always be in the plane of the ecliptic, because both the sun and the earth are always there.

The sun and the earth are both spherical bodies; if they were, therefore, of an equal size, the shadow of the earth would be cylindrical, as in plate 8, fig. 5; and would continue of the same breadth at all distances from the earth; and would, consequently, extend to an infinite distance, so that Mars, Jupiter, or Saturn, might be eclipsed by it; but, as the planets are never eclipsed by the earth, this is not the shape of the 'shadow; and, consequently, the earth is not equal in size to the sun.

If the sun were less than the earth, the shadow would be wider the farther it was from the earth, see plate 8, fig. 6; and would, therefore, reach to the orbits of Jupiter and Saturn, and eclipse any of these planets when the earth came between the sun and them; but the earth never eclipses them, therefore this is not the shape of its shadow; and, consequently, the sun is not less than the earth.

As we have proved that the earth is neither larger nor equal to the sun, we may fairly conclude that it is less; and, that the shadow of the earth is a cone, which ends in a point at some distance from the earth, see plate 8, fig. 7.