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as the shape of the wick of a candle may sometimes be seen through its flame, or the contents of a furnace in the midst of the brightest glare of it. But this, Dr. Herschel thinks, can only happen where the luminous matter of the Sun is not very accumulated.

From these appearances Dr. Herschel draws the following conclusions, that the Sun has a very extensive atmosphere, which consists of various elastic fluids, that are more or less lucid and transparent, and that the lucid one is that which furnishes us with light; that the generation of this lucid fluid on the solar atmosphere is a phenomenon similar to the generation of clouds in our atmosphere, which are produced by the decomposition of its constituent elastic fluids; but, with this difference, that the continual and very extensive decomposition of the elastic fluids of the Sun are of a phosphoric nature, and attended with lucid appearances, by giving out light. To the objection that such decomposition, and consequent emissions of light, would exhaust the Sun, he replies, that, in the decomposition of phosphoric fluids, every other ingredient except light may return to the body of the Sun; and besides, the exceeding subtilty of light is such, that in ages of time, its emanation from the Sun cannot very sensibly lessen the size of so great a body.

From the atmosphere, Dr. Herschel next proceeds to state that the body of the Sun is opaque, of great solidity, and its surface diversified with mountains and valleys; that the Sun is nothing else but a large lucid planet, evidently the first, or, strictly speaking, the only primary one of our system, al others being truly secondary to it. Its similarity to the other globes of the solar system, with regard to its solidity, its at mosphere, and its diversified surface, the rotation on its axis, and the fall of heavy bodies, lead to suppose that it is inhabited, like the rest of the planets, by beings, whose organs are adapted to the peculiar circumstances of that vast globe. Should it be objected that the heat of the Sun is unfit for a habitable world, he answers, that heat is produced by the Sun's rays only when they act on a calorific medium, and that they are the cause of the production of heat by uniting with the matter of fire which is contained in the substances that are heated. He also suggests other considerations intended to invalidate the objections, but they require more room to detail them than can be afforded in this work.

After Dr. Herschel thinks he has shown that the heat of n is not so great as to prevent it from being inhabited,

he then deduces from analogy a variety of arguments to confirm the notion of the Sun being a habitable body; and then infers, that if the Sun be capable of accommodating inhabitants, the other stars, which are suns, may be appropriated to the same use; and thus, says he, we see at once what an extensive field for animation thus opens to our view. The reader is referred to the Philosophical Transactions for 1795, where he will find many ingenious remarks and observations relating to this subject.

Dr. Wilson, late professor of astronomy, Glasgow, supposes the spots of the Sun are depressions, or excavations, rather than elevations, and that the dark nucleus of each spot is the opaque body of the Sun, seen through an opening in the luminous atmosphere with which he is surrounded. See Wilson: Philosophical Transactions, 1744 and 1783.

Various other hypotheses have been advanced, as to the cause of these spots, and the nature of the luminary on which they appear. Lahire and Laland suppose ose them to be eminences, or dark bodies like rocks, on the body of the Sun, appearing at times in consequence of the flux and reflux of the liquid igneous matter of the Sun. That part of the opaque rock which at any time thus stands above, gives the appearance of the nucleus, while those parts that lie only a little under the igneus matter appear to us as the umbra which surrounds the dark nucleus. Some other astronomers consider these spots as scoria floating in the inflammable liquid matter, of which they conceive the sun to be composed. Galileo, Hevelius, and Maupertius, seem all to have entertained this opinion. All these hypotheses are founded upon a supposition that the Sun being in itself a hot and luminous body; which opinion is contradicted by numerous and well established facts; for instance, on the tops of mountains of sufficient height, where clouds can seldom reach to shelter them from the direct rays of the Sun, we find regions of perpetual snow. Now, if the solar rays themselves conveyed all the heat we find on our globe, it ought to be hottest where their course is least interrupted, viz. on the tops of those mountains, which we know, from observation, to be in a constant state of congelation. The same has been observed by those who have ascended in balloons; that is, the higher they ascend the greater degree of cold they expepience: the Sun itself appears diminished both in splendour and magnitude, and the heavens, instead of the azure or blue, which we observe, approach more and more towards a total obscurity. These facts, to which might be added many others, are sufficient to explode the common notion of the Sun being a globe of fire, and to show at the same time that those planets which are nearest to the Sun, are not necessarily the hottest, nor those the coldest that are more remote; and hence, many of the fanciful calculations relative to light and heat experienced by the different planets of our system fall to the ground; as it is obvious, from what is stated above, that by certain modifications of the planetary atmospheres, the light and heat might be equalized throughout the solar system.

9. That luminous appearance, or faint light, which is sometimes seen, particularly about the time of the vernal equinox, a little before the rising or after the setting of the Sun, is called the zodiacal light.

It is the general opinion that this phenomenon is produced by the reflection of the Sun's atmosphere. The fluid which transmits the zodiacal light to us, according to LA PLACE, is extremely rare, since the stars are visible through it; its colour is white, and its apparent figure that of a cone, whose base is applied to the Sun. The length of the zodiacal light sometimes subtends an angle of more than 90°, but the atmosphere of the Sun does not extend to so great a distance, and cannot therefore reflect this light. La Place concludes that the true cause of the zodiacal light is still unknown.

QUESTIONS.

What is the Sun ?

What is the mean diameter of the Sun?

How many times is the Sun larger than the Earth?

What is the mean apparent diameter of the Sun, when is it greatest, &c. ?

Has the Sun an atmosphere, and what is its supposed height ?

What are the solar spots ?

In what time does the Sun revolve on its axis; what is its true figure; and in what angle is the solar equator inclined to the plane of the ecliptic? Has the Sun any other real motion, besides that

on its axis?

What are the apparent motions of the Sun? What is the cause?

What is the zodiacal light ?

CHAPTER IV

Of the Geocentric motions of the Planets, &c.

1. The most striking circumstance in the planetary motions, is the apparent irregularity of those motions; the planets one while appearing to move in the same direction among the fixed stars as the Sun and Moon; at another in opposite directions, and sometimes appearing nearly stationary.

These irregularities are only apparent, and arise from a combination of the motion of the Earth and motion of the planet; the observer not being conscious of his own motion, attributing the whole to the planet. The planets really move, as has already been observed, according to the order of the signs, in orbits nearly circular, and with motions nearly uniform, round the Sun in the centre, at different distances, and in different periodical times. The periodical time is greater or less, according as the distance is greater or less. Upon the hypothesis that the planets thus move, we can ascertain, by help of observation, their distances from the Sun, and thence compute for any time the place of a planet, which is always found to agree with observation. As the principal planets are always observed to be nearly in the ecliptic, and as they revolve round the Sun in orbits nearly circular; in order to simplify the illustration of their geocentric motions, we may, for the present, without any material error, consider them as moving uniformly in circular orbits, which coincide with the plane of the ecliptic.

2. The inferior planets, Mercury and Venus, are limited in their elongations from the Sun; the greatest elongation of Mercury being about 28°, and that of Venus 47°.

The interval of time between two successive inferior conjunctions can be observed; for, in inferior conjunction, the planet being nearest to the earth, appears largest, and may be observed with a good telescope, even a very short time before the conjunction. For our purpose here it is not necessary that the time of conjunction should be observed with great accuracy. Let T represent the time between two successive inferior conjunctions; onjunctions; then, to a spectator in the Sun, in the time T, the inferior planet, (moving with a greater angular velocity) will appear to have gained four right angles, or 360° on the Earth; and the planet and Earth being supposed to move with uniform velocities about the Sun, the angle gained (that is, the angle at the Sun be tween the Earth and planet, reckoning according to the order of the signs,) will increase uniformly.

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Let TEL represent the orbit of the Earth, DPGON that of an inferior planet, each being supposed circular, S the Sun in the centre, and P the place of the planet when the Earth is at E. Then in the triangle SEP we obtain the angle SEP the elongation by observation, and the angle PSE by computation; for it is the angle the planet has gained on he Earth since the preceding inferior conjunction. There

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