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to a comet whose progressive motion in that part of its orbit which is nearest to the sun, is more than equal to eight times the velocity of Mercury.

Charles. Were not comets formerly dreaded, as awful prodigies intended to alarm the world?

Tutor. Comets are frequently accompanied with a luminous train called the tail, which is supposed to be nothing more than vapour rising from the body in a line opposite to the sun, but which, to uninformed people, has been a source of terror and dismay, and to this opinion many of our poets have alluded:

-Where the train

Of comets wander in eccentric ways

With infinite excursion through th' immense
Of ether, traversing from sky to sky

Ten thousand regions, in their winding road,
Whose length to trace imagination fails;
Various their paths-

While distant orbs with wonder and amaze
Mark their approach, and night by night alarm'd
Their dreaded progress watch, as of a foe
Whose march is ever fatal, in whose train
Famine and war, and desolating plague,
Each on his pale horse rides, the ministers
Of angry Heaven to scourge offending worlds.

MALLET'S EXCURSION

CONVERSATION XLVI.

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 AUTUMN, line 1086.

James. You told us a few days ago, that the sun has a rotation on its axis; how is that known?

Tutor. By the spots on its surface, it is known that he completes a revolution from west to east on his axis in about twenty-five days, two days less than his apparent revolution, in consequence of the earth's motion in her orbit, in the same direction.

Charles. Is the figure of the sun globular?

Tutor. No; the motion about its axis renders it spheroidical, having its diameter at the equator longer than that which passes through the poles.

The sun's diameter is equal to 100 diameters of the earth, and therefore his bulk must be a million of times greater than that of the earth, but the density of the matter of which it is composed is four times less than the density of our globe.

We have already seen that by the attraction of the sun, the planets are retained in their orbits, and that to him they are indebted for light, heat, and motion:

Fairest of beings! first created light!

Prime cause of beauty! for from thee alone
The sparkling gem, the vegetable race,

The nobler worlds that live and breathe their charms,

The lovely hues peculiar to each tribe,

From thy unfailing source of splendour draw!
In thy pure shine, with transport I survey
This firmament, and these her rolling worlds
Their magnitudes and motions.

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CONVERSATION XLVII.

Of the Fixed Stars.

Tutor. We will now put an end to our astronomical conversations, by referring again to the fixed stars, which, like our sun, shine by their own light.

Charles. Is it then certain that the fixed stars are of themselves luminous bodies; and that the planets borrow their light from the sun?

Tutor. By the help of telescopes it is known that Mercury, Venus, and Mars shine by a borrowed light, for like the moon, they are observed to have different phases according as they are differently situated with regard to the sun. The immense distances of Jupiter, Saturn, and the Herschel planet, do not allow the difference between the perfect and imperfect illumination of their discs or phases to be perceptible.

Now the distance of the fixed stars from the earth is so great, that reflected light would be much too weak ever to reach the eye of an observer here.

James. Is this distance ascertained with any degree of precision.

Tutor. It is not: but it is known with certainty to be so great, that the whole length of the

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earth's orbit, viz. 190 millions of miles, is but a point in comparison of it; and hence it is inferred that the distance of the nearest fixed star, cannot be less than a hundred thousand times the length of the earth's orbit; that is a hundred thousand times 190 millions of miles, or 19,000,000,000,000 miles; this distance being immensely great, the best method of forming some clear conception of it, is to compare it with the velocity of some moving body, by which it may be measured. The swiftest motion with which we are acquainted is that of light, which, as we have seen, is at the rate of twelve millions of miles in a minute; and yet light would be about three years in passing from the nearest fixed star to the earth,

A cannon ball which may be made to move at the rate of twenty miles in a minute, would be 1800 thousand years in traversing the distance. Sound, the velocity of which is thirteen miles in a minute, would be more than two million seven hundred thousand years in passing from the star to the earth. So that if it were possible for the inhabitants of the earth to see the light; -to hear the sound;-and to receive the ball of a cannon discharged at the nearest fixed star; they would not perceive the light of its explosion for three years after it had been fired; nor receive the ball till 1800 thousand years had

* See Dr. Enfield's Institutes of Natural Philosophy, p. 347. Second edition, 1799.

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