actual discovery, by observing its action upon the planet Uranus.

Neptune is believed to be accompanied by two Satellites.

The light and heat of Neptune are 900 times less than at the Earth.

This planet was discovered under circumstances, which give a greater triumph to modern science, than any other discovery recorded in the annals of human knowledge. During several years previous, Uranus had been observed to deviate, in a mysterious manner, from the path assigned to it by the most careful calculations. Nothing but the supposition of another planet, existing somewhere in its vicinity, could account for the disturbance. Accordingly, two mathematicians, Mr. Adams, of England, and M. Leverrier, of Paris, undertook to calculate the position of this unknown planet. Both, though unknown to each other, arrived at conclusions differing but slightly. M. Leverrier, however, wrote to Dr. Galle, of Berlin, and requested him to direct his telescope to a certain point of the heavens. He did so, and the new planet was found within only one degree of the point specified by the mathematician.

CHAPTER XV.

THE ASTEROIDS.

THE Asteroids are a number of small bodies revolving between the orbits of Mars and Jupiter.

The Asteroids are distinguished from the other primary planets, by the following circumstances:

1. Their orbits have a much greater inclination to the ecliptic; that of Pallas, making with it an angle of 341 degrees.

By how many satellites is it accompanied? How much light and heat has it? What are the asteroids? How are they distinguished from the other planets?

2. Their orbits are much more eccentric, especially those of Juno and Pallas.

3. Their orbits, instead of being concentric, cross each other.

4. They revolve around the Sun, at very nearly the same distance from it, and in about the same time.

The magnitudes of these bodies have not been certainly ascertained; but they are supposed to be all smaller than the Moon.

Their average distance from the Sun, is about 260 millions of miles.

Their annual revolution is performed in about 4 years.

All the Asteroids have been discovered during the present century. Ceres, Pallas, Juno, and Vesta, between 1801 and 1807; and the others since 1845.

CHAPTER XVI.

APPARENT MOTIONS OF THE HEAVENLY BODIES.

ALL the heavenly bodies collectively, have two apparent revolutions around the earth; an Annual revolution rom west to east, and a Daily revolution from east to west.

The first of these is caused by the Annual revolution of the Earth, and the second by its Diurnal rotation.

What are their magnitudes? What is their average distance from the sun? What is the time of their annual revolution? When were they discovered? What apparent revolutions have the heavenly bodies collectively? How are these caused?

The heavenly bodies rise in the east and set in the west, once every 24 hours, in consequence of the rotation of the earth on its axis.

The planets individually have two apparent motions: viz., Direct and Retrograde.

A planet's motion is said to be Direct, when it appears to move from west to east, according to the order of the Signs.

A planet's motion is said to be Retrograde, when it appears to move backward, from east to west, or contrary to the order of the Signs.

When a planet appears to remain for some time, in the same point of the heavens, it is said to be Stationary. These appearances are caused by the real motion of the Earth, together with that of the planet.

A planet appears to be Stationary, when it is moving directly towards, or from the Earth.

In the annexed diagram, supposing the earth to be at rest at E, when Venus moves from V to V', 'she appears to move from D to B, westward or contrary to the order of the signs; and when she passes from V' back again to V, she appears to move from B to D, or according to the order of the signs. But instead of being at rest, the earth is moving the same way. Suppose, therefore, she moves from E to E', while Venus is moving from V to V'; when Venus arrives at V', she will have appeared to pass from D to B', in the same direction as before, only not so far by the arc B B'. If the earth moves from E to E', while Venus is moving from V' to V, Uranus will appear to pass from B to D', in the same direction as when the earth was at rest, only farther by the arc DD'. The earth's motion, therefore, does not affect the apparent direction of an inferior planet's motion, but makes it slower as it passes through its inferior, and quicker as it passes through its superior conjunction. The apparent motion

Why do the heavenly bodies rise and set? What apparent motions have the planets individually? When is a planet's motion said to be direct? When is it said to be retrograde? When is a planet said to be stationary? By what are these appearances caused? When does a planet appear to be stationary ?

of an inferior planet is therefore retrograde, while passing through its inferior, and direct while passing through its superior conjunction. Were the earth at rest, the planet would appear stationary at the points of greatest elongation, viz., at V and V'; but, owing to the earth's motion, this happens at a point between inferior conjunction and greatest elongation.

The apparent motions of a superior planet, are somewhat different from those of an inferior planet, because the earth is moving faster than the planet; but they may be explained in a similar manner. In passing through its superior conjunction, the motion of a superior planet is direct, because the combined effect of the earth's and planet's own motions, is to give it such a motion; but in opposition, the earth moving more rapidly, passes the planet, and gives it an apparent retrograde motion. Thus, when Mars is moving from M to M', were the earth at rest, at E, it would appear to move from B to A, while the effect of the earth's motion is only to give it a greater apparent motion in the same direction. From Q to Q', it is direct or retrograde, according as the effect of the earth's motion is greater or less than that of the planet. When these effects are the same, the planet appears stationary, which happens between quadrature and opposition.

CHAPTER XVII.

ECLIPSES.

AN Eclipse is the concealment of a heavenly body, by some opaque body, intercepting the Sun's rays. The principal Eclipses, are the Solar and Lunar. A Solar Eclipse, is an eclipse of the Sun.

It is caused by the Moon's passing between the Earth and the Sun, and concealing the Sun from our view. A Lunar Eclipse, is an eclipse of the Moon.

It is caused by the Moon's passing through the Earth's shadow.

A Solar Eclipse can happen only at New Moon; a Lunar Eclipse, at Full Moon.

Eclipses are of two kinds, Total and Partial.

A Total Eclipse is one, in which the disc is entirely concealed.

A Partial Eclipse is one, in which only a part of the disc is concealed.

An Annular Eclipse is an eclipse of the Sun, in which a bright ring appears around the dark body of the Moon.

It happens when the Moon is too far from the Earth, to conceal the whole of the Sun's disc.

What is an eclipse? What are the principal eclipses? What is a solar eclipse? How is it caused? What is a lunar eclipse? How is it caused? When can these eclipses happen? Of how many kinds are eclipses? What is a total eclipse? What is a partial eclipse? What is an annular eclipse? When does it happen?