But besides the motion which every planet has round the Sun, it has also another motion which remains to be noticed. If a ball be thrown from the hand, the projectile force causes it to move straight forward; but it also spins, or revolves about its own centre, and a line through the centre around which it moves, is termed the axis. Just so is it with regard to the planets; each moves forward in its orbit, and at the same time around its own axis. The axis of a revolving body, therefore, is an imaginary line passing from one part of its surface to the opposite part, through the centre around which it moves -But neither the Sun, nor any one of the planets whose axes have been discovered, revolves with its axis upright, or at right angles to its orbit, but always more or less inclined to it. Thus the axis of the

deg. min. Sun is inclined to its own orbit 82 44

The Earth completes the revolution round its axis in 24 hours; that is, every part of the carth, once in that time, turns round toward the sun; 24 hours is the period of our solar day, which is completed by this revolution.

ho. min. sec.

The length of the day to Mercury is.... 24 5 0

Still a few words on the subject of attraction or grafor it would be unfair to quit it without saying

vitation;

that an opponent has lately arisen to the theory of Newton in the person of Wm. Frend, a mathematician of considerable eminence, and the author, among other works, of a valuable little volume published annually under the title of Evening amusements, or the Beauties of the heavens' displayed.' In the volume for this year this gentleman, assuming the features of a Democritus, of a laughing, or more strictly speaking, of a sneering philosopher, decries Newton's doctrine; and if you can be laughed out of conceit at what he terms that amusing romance which goes under the name of the principles of Natural Philosophy, by Sir Isaac Newton,' it is but fair that you should have the opportunity. He says that it is just as worthy of belief as Moore's Almanack,' and affects to think the senseless diagram published in that almanack, of equal value with the diagrams of Newton.

He says that Newton supposes a particle to be placed at a distance from a globe, composed of particles, --and the particle without, as well as within the globe, is supposed to be endowed with a power of attraction, or drawing every particle to itself.' In illustration of this, as I presume, he further says that, as matter according to the Newtonian opinion is infinitely divisible, the smallest insect, that is creeping on the smallest object perceptible to our senses, has a power, independent of that necessary to its existence, which is exerted on objects as small as itself on the planet Herschel,' or the Georgium Sidus.

Now, (in the words of the same author) all this is very well, and the problem is amusing to the mathematician; but what does it tend to?' I answer that this ingenious gentleman knows very well that irony will sometimes prevent the admission of the best established truth, when that truth is not obviously

tangible. He acknowledges that the about the Sun, but he also says that,

planets revolve we may not be able to comprehend the whole of the wonderful system in which we are placed; yet, the confession of ignorance is better than the introduction of causes, which have not a foundation in nature.

If this philosopher, in attempting to ridicule the doctrine of Gravity, had told us why, when the insect of which he speaks, falls, it falls to the earth; and why another insect on the opposite side of the globe, at our antipodes, falls to the earth also. If he had shewn some more reasonable cause why they do not both fall off into space, then we should have had a reasonable sort of opposition to the doctrine of gravity. But since both these insects fall in opposite directions to the Earth, there must be a cause, and let him tell us what that cause is, if it be not gravity?

It is true that, according to the doctrine of Newton, all the particles of matter possess this power; and it may be true that every particle gravitates to every other particle; yet when it is asserted that, according to this theory, the smallest insect creeping on the earth has a separate attraction for the smallest individual insect creeping on the Georgium Sidus, we must be careful not to admit a proposition, put in as ridiculous a shape as possible, as an argument against a theory so perfectly established; * we must entertain it with a smile, since it was intended as a sneer.

* When it is asserted that the truth of this theory is perfectly established, it is so asserted upon undoubted facts, not regarding the earth alone. In treating on the tides, we shall find that their periods are consonant with the motions of the moon, which therefore must attract the earth. In winter the earth moves quicker than in summer, because in winter it is nearest to the sun. It is

What, but the invisible power of attraction, caused the balls to approach each other in the experiment of Cavendish, or the plumb line to deviate from the perpendicular towards the mountain of Schihallien, but the attraction existing in the two bodies for each other? What is it that prevents loose rocks and stones on the surface and at opposite sides of the earth from falling away from it but attraction? What other power causes the sea to keep its bed on opposite sides of the globe, and vessels to sail with the keels towards each other? What but attraction causes the inhabitants of the opposite side of the earth to walk with their feet towards our feet? When all these facts can be explained by some more plausible theory, than by supposing that the same cause pervades the whole system which we know to exist in every part of the earth, but which, if I understand the author of Evening Amusements, he denies to have any foundation in nature, we shall be glad to listen to him, and to sneer with him at the doctrine of Newton, and at the laborious and useless calculations of many able mathematicians, all of which tend to confirm its truth, and not one of which has he shewn to be fallacious. Meantime, we will take his advice, we will examine for ourselves, and not prostrate our understandings to any (one) man's authority', in opposition to a host of confirming evidence.

certain that the other planets move around the Sun, moving quickest when nearest to it; and that the planets when nearest to each other, affect and disturb one another's motions (p. 35). It is undoubtedly theoretical when it is said that it is attraction which causes these effects; but we hazard nothing in asserting the truth of the theory, since all the planetary motions are consonant with the laws that have been recited; and since it is certain that the power of attraction extends 240,000 miles; namely, to the moon.

LECTURE III.

Of the individual members of the solar system-The Sun, the Planets, and Comets.

OF THE SUN.

ALL astronomers agree in describing the Sun as a nearly spherical body, the prime source and dispenser of light and heat to the whole system of bodies revolving around it.

this very

sub

A person wholly uninstructed, feeling these beneficial effects, would doubtless conclude the sun to be an immense globe of fire; and this long continued to be the prevailing opinion. Though it may be impossible to disprove this notion, we must, if we adopt it, also adopt the belief that light and heat are the same, or that one is a modification of the other; and upon ject philosophers have not yet agreed. If the sun be actually a body of fire, it is subject to some phenomena, not altogether consistent with our notions of fire alone, or when considered separately from some combustible substance; for, on the surface of it frequently appear dark spots, more or less in number, which if they belong to the peculiar nature of the mass, cannot be mere fire. The existence of these spots was not known until since the discovery of telescopes, not until the year 1611, and the honour of the discovery was claimed by two astronomers, Galileo and Scheiner. To us it is unimportant