as may be seen by reference to fig. 7, Plate II., where the orbits of three of the six known comets out of hundreds which, in the course of ages, have made their appearance, are laid down proportionally. It will be observed that they sometimes approach the sun, and in other parts of their orbits recede from him an immense distance. Some of these singular bodies do not revolve round the sun at all; but, making their first appearance at one part of the system, they sweep towards the sun; then leaving him, they dart across to the opposite quarter, and are seen no more. Either they are lost, or dissolve in the immensity of space; or they visit in the same manner a succession of other systems in remote regions of space.

The planets resemble the earth, as we shall hereafter shew more at length, in various particulars, more especially in their being opaque and solid bodies; but these singular visitants, the comets, appear to have nothing in common with either the planets or the sun, but to consist entirely of the most rare and transparent vapour, far more delicate than the most fleecy cloud that floats in our atmosphere.

5. Instruments called orreries are frequently made use of to explain the various movements of the solar system; and it may be allowed that they illustrate some celestial phenomena tolerably well. On other points, however, they convey very incorrect notions, and are calculated to mislead the student in estimating the relative sizes and distances of the sun and planets. The earth, for instance, is usually represented by a globe of an inch and a half in diameter, while the orbit of Uranus is performed at not more than eighteen inches from

the sun.

How very little these proportions accord with the true, will be at once seen from the following calculations.

Let the earth be represented by a globe 14 inches in diameter; the proportionate diameters of the other planets would be as follows:

While the sun would be represented by a massive globe whose diameter would be fourteen feet.

Preserving the same scale for the distances of the planets from the sun, they would be:

The diagram of the solar system (fig. 7, Plate II.) exhibits the orbits of the planets, as well as of three known comets, proportionally delineated.

6. The sun, in his annual motion through the heavens, describes a path among the fixed stars, which has received the name of the ECLIPTIC. The phenomena

of the annual revolution of the earth will be the same, whether we suppose it to be at rest, and the sun to revolve round it, with the ancients; or whether we assume the sun to be the fixed, and the earth the revolving body. The complete revolution of the sun among the stars forms the year, which is equal to 365-2563612 of our natural days, or revolutions of the earth on its axis.

SECTION II.

THE SUN.

THE SUN-THE SOLAR SPOTS-ROTATION OF THE SUN ON HIS AXISVARIATION IN HIS DIAMETER, PROVING THE EARTH'S ORBIT TO BE AN ELLIPSE-PROOF THAT THE SUN, AND NOT THE EARTH, IS THE CENTRAL BODY-ABERRATION OF LIGHT-ZODIACAL LIGHT.

7. In this part of the work will be introduced such particulars respecting the form, physical constitution, or any other peculiarity of the heavenly bodies, as the light of science has disclosed to us at the same time we shall not hesitate to refer, under the head of Descriptive Astronomy, to any fact relating to their mutual connection with each other through the attraction of gravity, which may, however, be more fully illustrated under the head of Physical Astronomy.

This division of the work will more particularly shew how much the moderns are indebted to the telefor the revelation of truths which without its aid would have remained unknown.

scope

8. When contemplated with respect to their physical constitution, the heavenly bodies may be divided