companied with a luminous train called the tail, which is supposed to be some electric phenomenon issuing from the body in a line opposite to the sun, but which, to uninformed people, has been a source of terror and dismay.

James. Do comets shine by their own light?

Tutor. It was, till within these few years, supposed that comets borrowed all their light from the sun, but the appearance of two very brilliant comets, of late, seems to have overturned that theory. One of these was visible, for several weeks, in 1807, and the other from September to the end of the year 1811. Of the former, Dr. Herschel has given an elaborate account in the 98th vol. of the Philosophical Transactions. Previously to the appearance of these, it was generally supposed that the light of comets, like that of

the moon and planets, was reflected light only. A new theory is now adopted by Dr. Herschel, and other eminent astronomers, who have had capital opportunities, in both the instances referred to, for accurate observations. Dr. Herschel says, with respect to the comet in 1807, "we are authorized to conclude, that the body of the comet, on its surface, is self-luminous, from whatever cause this quality may be derived. vivacity of the light of the comet, also, had a much greater resemblance to the radiance of the stars, than to the mild reflection of the sun's beams from the moon."

The

The same inference has been drawn from the observations made on the comet of 1811, which distinctly exhibited, to very powerful telescopes, the several parts of which the comet is composed.

Charles. What are those parts? Tutor. They are the nucleus, the head, the coma, and the tail.

The nucleus is a very small, brilliant, and diamond-like substance in the centre, so small as to be incapable of being measured.

The head includes all the very bright surrounding light; inferior telescopes, that will not render the nucleus visible, are often able to exhibit the head thus described.

The

head of the comet of 1807 was ascertained to be 538 miles in diameter: that of 1811 to be about the size of the moon.

The coma is the hairy or nebulous appearance surrounding the head.

The tail, which, in some comets, extends through an immense space, it is thought may be more satisfactorily accounted for, by supposing it to consist of radiant matter, such as the

matter of the aurora borealis, than when we unnecessarily ascribe the light to a reflection of the sun's illuminations thrown upon vapours supposed to arise from the body of the comet. The tail of the comet, in 1807, was ascertained to be more than nine millions of miles in length; and that in 1811 was full 33 millions in length. James. Was this comet at a great distance from the earth?

Tutor. On the 15th of September, its distance from the sun was more than 95 millions of miles; and its distance from the earth, at the same time, was upwards of 142 millions of miles.

After all the exertions of astronomers of all countries, there is no class of celestial objects whose theory is so little advanced as that of comets; we will, therefore, dwell no longer upon

it.

CONVERSATION XXV.

Of the Sun.

TUTOR. Having given you a particular description of the planets which revolve about the sun, and also of the satellites which travel round the primary planets as central bodies, while they are carried at the same time with these bodies round the sun, we shall conclude our account of the solar system by taking some notice of the sun himself:

Informer of the planetary train,

Without whose quick'ning glance their cumbrous orbs

Were brute unlovely mass, inert and dead,
And not, as now, the green abodes of life.

THOMSON'S AUTUмn, line 1086,