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a parallax of 8.92" is about 91,350,000 miles. Hansen has contributed something towards the elucidation of the matter, which must not be passed over. As far back as 1854, this distinguished mathematician expressed his belief that the received value of the solar parallax was too small, and in 1863 he communicated to the Astronomer Royal a new evaluation, derived from his Lunar theory by the agency of the co-efficient of the parallactic inequality. The result is 8.9159", a quantity fairly of accord with the other values set forth above. Some astronomers may hope that this new determination may not be confirmed, as it will require a total recomputation of every numerical quantity (and they are legion) involving the Sun's distance as a unit, without any real practical advantage arising therefrom.

Having ascertained the true mean distance of the Earth from the Sun, it is not difficult to determine by trigonometry the true diameter of the latter body, its apparent diameter being known from observation; and, as the most reliable results show that the Sun at mean distance subtends an angle of 32′ 42′′, it follows that the true diameter is 846,620 miles. It is generally accepted that there is no compression. The surface of this enormous globe therefore exceeds that of the Earth 11,450 times, and the volume 1,228,020 times; since the surfaces of spheres are to each other as the squares of the diameters, and the volumes as the cubes.

The lineal value of 1" of arc at the distance of the Sun is 448 miles.

The Sun's mass, or attractive power, exceeds that of the Earth 319,500 times, and (approximately) is 674 times the masses of all the planets put together.

By comparing the volumes of the Sun and the Earth and bringing in the value of the mass, we obtain the relative specific gravity or density of the two.

1.

The Sun's volume exceeds the Earth's by 1,228,020 to 1; the Sun's mass exceeds the Earth's in the lesser ratio of 319,500 to Therefore the density of the Sun is to the density of the Earth as 319,500 to 1,228,020, or 1 to 3.8. Then taking Baily's value of the density of the Earth (5.67 times that of water) we are enabled to find the density of the Sun to be 147 times that of water.

This consideration of the comparative lightness of matter composing the Sun, has led Sir J. Herschel to think that it is highly probable that an intense heat prevails in its interior, by which its elasticity is reinforced, and rendered capable of resisting [the] almost inconceivable pressure [due to its intrinsic gravitation] without collapsing into smaller dimensions."*

* Outlines of Ast. p. 297.

We thus see that the Sun is eminently worthy of the important position it holds as the centre of our system, and thus early it will be appropriate to mention that the Sun is to be regarded as a fixed body so far as concerns ourselves; therefore, when we say that the Sun "rises," or the Sun "sets," or the Sun moves through the signs of the zodiac once a year, we are stating a conventional untruth; it is we that move and not the Sun, the apparent motion of the latter being an optical delusion.

*

The Sun is a sphere, and is surrounded by an extensive and rare atmosphere, and is self-luminous, emitting light and heat which are transmitted certainly beyond the planet Neptune, and therefore more than 2,700 millions of miles. As regards the Sun's heat, it has been calculated that only 2381000000 reaches us, so what the whole amount of it must be, passes human comprehension, like many other things in science. Our annual share would be sufficient to melt a layer of ice all over the Earth 38 yards in thickness, according to Pouillet.† Another similar calculation determines the direct light of the Sun to be equal to that afforded by 5,563 wax candles of moderate size, supposed to be placed at a distance of one foot from the observer. The light of the Moon being probably only equal to that of one candle, at a distance of 12 feet, it follows that the former exceeds the latter 801,072 times, according to Wollaston.

When telescopically examined, the surface of the Sun is frequently found to be dotted over with dark spots or maculæ,‡ each surrounded by a fringe of a lighter shade, called a penumbra,§ the two not passing into each other by graduations of tints, but abruptly. In the few cases in which a gradual tint has been noted, Sir J. Herschel believes the circumstance may be ascribed to an optical illusion, arising from imperfect definition on the retina of the observer's eye. This, however, is not always the case, several spots being occasionally included within the limits of one penumbra. And it may further be remarked that cases

*Ganot, Physics, p. 331. This was calculated on the old value of the solar parallax. I have not altered it.

† Cited by Ganot, as above. To show the great power of the calorific rays of the sun, I may mention that in constructing the Plymouth breakwater, the men, working in diving bells, at a considerable distance below the surface, had their clothes burnt by coming under the focus of the convex lenses placed in the bell to let in the light.

Lat. macula, a blemish. Sir J. Herschel upholds a further classification: he applies to the ordinary black central portions the term umbra (shadow), on the highly probable ground that the blackness is mainly relative. Patches of deeper blackness are occasionally noticed in the umbræ; Sir John limits to these the designation nucleus, sometimes indiscriminately applied to all the blackish area.

§ Pene, almost, and umbra, a shadow.

of an umbra without a penumbra, and the contrary, are on record, though these may be termed exceptional, and considered as closely relating to material organic changes just commencing or terminating. A marked contrast subsists in all cases between the luminosity of the penumbra and that of the general surface of the Sun contiguous. The outlines of the penumbræ are irregular to the utmost extent, but the umbræ, especially in the larger spots, are often of regular form (comparatively speaking, of course), and the nuclei of the umbræ still more noticeably partake of a compactness of outline. They are for the most part confined to a zone extending 35° or so on each side of the solar equator, and are neither permanent in their form nor stationary in their position, frequently appearing and disappearing with great suddenness. The multitude of facts concerning solar spots, accumulated from the journals of many observers, extending over long periods of years, is so great as to bewilder one, and my present desire to marshal these in a suitable manner will be rather a difficult task, and when performed, it is nearly certain that something of importance will have been left out.

The general limit of the spots in latitude may be stated as above at 35°, but instances of spots seen beyond this are on record. In 1858, Carrington saw one 44° 53' distant from the solar equator; in 1825, Capocci one, 46°; in 1846, C. H. Peters one, 50° 55'; and La Hire, in the last century, one having a latitude as much as 70°. They are confined to two belts on either side of the Sun's equator, rarely if ever being seen directly under the equator, or nearer to it than 8° of north or south latitude: from 8° to 20° is their most frequent range. They are always more numerous and of a greater general size in the northern hemisphere; the zone between 11° and 15° north is ticularly noted for large and enduring spots. A gregarious tendency is very obvious, and where the groups are very straggling, the longer line joining extreme ends will pretty generally be found more or less parallel to the equator, and not only so, but extending across nearly the whole of the visible disc.

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Sir John Herschel remarks, 'These circumstances point evidently to physical peculiarities in certain parts of the Sun's body more favourable than in others to the production of the spots, on the one hand; and on the other, to a general influence of its rotation on its axis, as a determining cause in their distribution and arrangement, and would appear indicative of a system of movements in the fluids which constitute its luminous surface; bearing no remote analogy to air trade-winds from whatever cause arising.' In reference to the distribution of the spots in lati

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*Outlines of Ast. p. 251.

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tude, the recent observations of Carrington have placed us in possession of some important facts. That observer found that as the epoch of minimum approached, the spots manifested a very distinct tendency to advance towards the equatorial regions, deserting to a great extent their previous haunts above the parallels of 20° or so. After the minimum epoch arrived, a sudden and marked change set in, the equatorial regions becoming almost deserted by the spots, which when they reappeared showed themselves chiefly in parallels higher than 20°. Wolf finds that the observations of Böhm reveal the fact that the same peculiarity was noticed by that observer in the years 1833-6. Whether this is a general rule, remains yet to be ascertained, but Sir John Herschel remarks that if such be the case, "it cannot but stand in immediate and most important connection with the periodicity itself, as well as with the physical process in which the spots originate."

(To be continued.)

PERIODIC COMETS.

(Continued from vol. iii. p. 292.)

PONS'S COMET,

No. 3, was discovered by M. Pons on June 12, 1819. Professor Encke assigned to it a period of 5 years, which, as the table will show, was a very close approximation to the truth. It was not, however, seen from that time till March 8, 1858, when it was detected by Winnecke, at Bonn, who soon ascertained the identity of the two objects.

BRORSEN'S COMET,

No. 4, was detected by M. Brorsen, at Kiel, on February 26, 1846. The observations showed an elliptic orbit, and the epoch of the ensuing arrival at perihelion was fixed for September 26, 1851. But its position then was not very favourable, owing to its proximity to the sun, and it escaped observation. Bruhns rediscovered it on March 18, 1857. To the writer, on March 23, it possessed the usual nebulous appearance common to these objects, and had a diameter of about 2',though unfavourably placed in the morning twilight, which probably marred its brilliancy.

BIELA'S COMET,

No. 5, is another very remarkable periodic comet, hardly less interesting than Encke's; so we shall recapitulate its history at some length.

On March 8, 1772, Montaigne, at Limoges, discovered a comet in

Eridanus, which, from want of suitable instruments, he was unable properly to observe or see at all beyond the 20th; Messier, however, saw it four times between March 26 and April 3.

On November 10, 1805, Pons discovered a comet, which was found also by Bouvard on the 16th. It had a nucleus, and the diameter of the coma on November 23 was 6' or 7'. On December 8 it was nearest to the earth, and Olbers saw it without a telescope. Bessel and others calculated elliptic elements, and its identity with the comet of 1772 was suspected, though no predictions as to its next return were ventured on.

On February 27th, 1826, M. Biela, at Josephstadt, Bohemia, discovered a faint comet in Aries, which Gambart found on March 9. The observations extended altogether over a period of eight weeks, and it was soon made evident that the orbit was an ellipse of moderate eccentricity; and farther, that the comet was the same as had already been observed in 1772 and 1805.

In anticipation of its next return in 1832, investigations into the orbit of the comet, and the perturbations by which it would be affected, were undertaken by Santini, Damoiseau, and Olbers. Santini found that its period in 1826 was 2,455 days, but that the attraction of the Earth, Jupiter, and Saturn would accelerate its next return by rather more than 10 days, which he accordingly fixed for November 27, 1832. Damoiseau's investigations gave a similar result. Early in 1828, Olbers called attention to the fact that in 1832 the comet would pass within 20,000 miles of the earth's orbit; but that as the earth would not reach that particular point till one month after the comet had passed it, no danger was to be apprehended. Astronomers were quite satisfied as regards this matter, but not so the world at large, who were greatly alarmed lest a collision should take place, and our globe become the sufferer thereby.

Punctually at the time appointed the comet returned to perihelion, through which it passed within 12 hours of the time fixed by Santini five years previously. It was first seen at Rome on August 23, but, owing to its excessive faintness, it was not generally observed till two months later.

The next return was calculated to take place on July 23, 1839, but in consequence of its close proximity to the sun, the comet was not detected.

Continuing his researches, Santini fixed on February 11, 1846, as the epoch of the next perihelion passage; and as it would be visible for a considerable period, much interest was excited amongst astronomers, who anticipated that a remarkably good opportunity would be afforded for correcting the theory of its motion.

Di Vico, at Rome, discovered it on November 28, 1845, with the powerful telescopes at his command, and Galle, at Berlin, saw it two days later; but by the generality of observers it was not seen till the second or third week in December. I have already adverted to the very curious phenomenon which took place at this apparition of Biela's

comet.

The comet returned again to perihelion in September 1852, and was visible for three weeks. The same reason which prevented it being seen in 1839, also caused it to pass undetected in May 1859; so we

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