cleus visible, are often able to exhibit the head thus described.

The head of the comet of 1807, was ascertained to be five hundred and thirty-eight miles in diameter; that of 1811, to be about the size of the moon.

The term coma you have already explained, and the term tail is obvious. But what may have been the length of the tail of a comet?

That of 1807, was ascertained to be more than nine millions of miles in length; and that in 1811 was full thirty-three millions. O my dear child! mark the immensity of creation; for they pursued their course undisturbing, and undisturbed, and occupied apparently an insignificant portion of space.

But you were, my dear momma, to inform me, if comets possess heat, independent of the

sun.

If comets are "self-luminous," it is probable they do; one beneficial effect, which Sir Isaac Newton conjectures, comets may afford, is that of recruiting the sun with fresh fuel, and repairing the consumption of light and heat, by the streams continually sent forth in every direction from that luminary.

But let them be supposed not to possess heat independent of the sun, and then what degree of

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heat might they acquire by their near approach to the sun?

The comet, seen by Sir Isaac Newton, in the year 1680, was observed to approach so near the sun, that its heat was estimated by that great man to be 2000 times greater than that of red-hot iron.

Will you give me any idea, how long a body thus heated, would retain its heat?

A red-hot globe of iron of a single inch in diameter, exposed to the open air, will scarcely lose its heat in an hour; and it is said that a globe of red-hot iron, as large as our earth, would scarcely cool in fifty thousand years *.

* See Enfield's Institutes of Natural Philosophy, p. 296, 2d edition.

CHAPTER XI.

CONCLUSION.

Before I withdraw your attention from the solar system, I wish to press upon you five or six important inquiries.

We see stationed in this system, a stupendous globe of luminous matter, and observe revolving around it, opaque planets with their accompanying dark satellites; but the observations we have just made on comets show us, that there is nothing which renders such an arrangement in nature unavoidable, nothing that requires that the body which is stationary should be on fire, while the revolving bodies should be dark, and cold. How shall we account for this disposition of the planets? why not all light, all dark? why not a small dark body at the centre? and, if light must exist somewhere, why should we not find it at the confines of our system?

Mamma, how great would be the inconvenience and the misery of such an arrangement! It would be as bad as our having our eyes placed at our fingers' ends.

But why should the planetary bodies be so admirably adjusted, that the equatorial parts do

not become the polar, or the polar the equatorial?

This cannot, I am sure, be the result of some lucky accident; but the cause I am unable to explain.

The fact is, that, out of the infinite number of axes of rotation, upon which the planets might by a random stroke have accidentally fallen, the best of all has been selected; for, while the number is infinite that might have been wrong, those axes only, which pass through the longest, or the shortest diameters, can be permanent. Now, it is delightfully satisfactory to observe, that, while choice was restricted to such narrow limits, it has in all instances exhibited its perfection, by its selection of the shortest, and consequently of the most permanent axis of rotation.

Pray tell me something more: this is the very information I am so anxious to possess.

Well, then, there is a certain very simple law impressed upon bodies, which is truly wonderful in its effects. The law is, that all bodies shall continue in the state in which they are, whether of rest or of motion. If in motion, they shall go on with the same velocity, and in the same line, on which they were proceeding, till something shall change their course.

Mamma, is this the law, and are you now going to tell me its effects?

I am it is most curious; for it is by virtue of this law that attraction never has brought, nor never can bring, the planets to unite at a centre, but will keep them for ever circulating. How beautiful!

But what is particularly worthy of observation, is the admirable nicety with which these two forces balance each other. Why was not the centripetal force greater; why was it not less; why did it exist at all? Why was not the power of attraction greater? why was it not less? or why did this exist at all?

I suppose, mamma, had the attractive force varied greatly, the human species could not have existed. If it had had but little influence, worlds would have been running on to their own destruction; if it had had a greatly superior power, it would have buried us in its bosom.

Very right: but, you must observe, that, as the power of attraction exists in all bodies, of course it exists in each of the planets, and in each of their satellites, as well as in the sun: hence, each separate body of which our system is composed, attracts, and is attracted; therefore, the antecedent probability of their disturbing each other, in their revolutions, becomes very great.

And is every thing arranged with so much care, as to prevent even this?

Yes; to prevent every thing but periodical