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“ I have seen these rice-like particles on two occasions since, but not so well as on the first day, when the definition was exceedingly good. Yesterday (Feb. 24) I saw them for a few minutes, but with great difficulty. I use the full aperture, 121 inches, and a low power. On the first day I saw them (end of January 1864] I called Mr. Dunkin's attention to them. He appears to have seen them, and considers the figure above to represent them fairly. He says, however, that he should not have noticed them if his attention had not been called to them i."

A valuable synopsis of the question was presented to the Royal Astronomical Society in 1866 by Hugginsk. The following is a brief summary of its contents :

1. Granule is the best word to describe the luminous particles on the Sun's surface, as no positive form is thereby implied.

2. The granules are seen all over the Sun, including (occasionally) the surfaces of umbræ and penumbræ. More rarely they can be detected in faculæ. · 3. With low powers “rice grains" is a very suitable expression for these granules, but the regularity implied in this designation disappears to a great extent under high magnifiers. There is, however, undoubtedly, a general tendency to an oval contour.

4. The average size of the more compact granules is 1", of those more elongated 1.1", a few might be 3", many less than 1". They appear to be not flat discs, but bodies of considerable thickness.

5. The granules are sometimes packed together rather closely in groups of irregular and straggling outline ; at other times they are sparsely scattered. The well-known “mottling” arises wholly from the latter species of combination.

6. The Sun's surface is by no means uniformly level The whole photosphere appears corrugated into irregular ridges and vales, and the granules are possibly masses of rather dense cloudlike matter floating about in the photosphere, considered as composed of more aëriform matter. If the granules really are incandescent clouds, their general oval form inay be due to the influence of currents.

" Proceedings of Manchester Lit, and Philos. Soc., vol. iji. p. 250, 1864.

Month. Not., vol. xxvi. p. 260. May 1866.

The accompanying figure [28] shows some of the most characteristic modes of grouping of the bright granules noticed by Huggins on different occasions and on various parts of the Sun's surface, brought together, however, in one woodcut for convenience of comparison.

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IDEAL VIEW OF THE “GRANULAR" STRUCTURE OF THE SUN. (Huggins.)

Huggins has called attention to the fact that Janssen's photographs of 1877 disclose, amongst other important features, a frequent tendency of the granules to arrange themselves in a spiral form, accompanied by more or less loss of distinctness of outline of the individual granules. The same observer has put on record the fact that a similar appearance was noticed by himself as long ago as 1866:

“Saw distinctly the granules. A spiral band of closely associated granules, ending in one of larger size (fig. 26). In one area near the centre of the Sun's disks the granules appeared more elongated than usual (fig. 30), rather sparsely scattered, and the larger diameters very nearly in the same direction. In neighbouring area, the granules smaller and less elongated. Amongst these no general direction was observed !.” Fig. 29.

Fig. 30.

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SOLAR GRANULES 1866, SHOWING CYCLONIC

ARRANGEMENT. (Huggins.)

The present state of our knowledge respecting the physical constitution of the Sun, stated as shortly as possible, is, that the central solid or gaseous body of the Sun is surrounded by a series of concentric envelopes, the order of which reckoning outwards is as follows:

(1) The photosphere, the visible source of the solar light which reaches the Earth, defined by Young as a “shell of luminous clouds formed by the cooling and condensation of the condensible vapours at the surface where exposed to the cold of outer space.”

(2) The chromosphere, a thin casing of self-luminous gaseous matter, chiefly hydrogen gas in an incandescent state, and the seat of the solar prominences (formerly known as the “red flames” and seen only during total eclipses of the Sun until Lockyer and Janssen independently in 1868 conceived the idea that they might be rendered visible irrespective of the Sun being eclipsed).

1 Month. Not., vol. xxxviii. p. 102. Jan. 1878.

(3) The corona, a vast shell of unknown vapours in a highly attenuated state, many thousands of miles thick, and oberved to extend to at least 1° from what is ordinarily taken to be the visible edge of the Sun.

Tacchini arrived at the following general ideas from observations made by him on 281 days during 1880.

As to the distribution of solar phenomena over the Sun's surface: The spots remain near the equator and present two maxima between the parallels 10o and 20° on either side. At the equator they are rare, or wholly absent. Faculæ always occur at the equator ; they show maxima between + 20° and +30°, and come nearer the poles than the spots. Protuberances are rare near the equator; they present two principal maxima between +50° and +60°, and two secondary ones in the latitudes of the faculæ maxima. They reach further from the equator than the faculæ, but the polar caps remain free of them. Of the two hemispheres the northern showed, during 1880, the greater activity.

To the cloudy stratum giving rise to the penumbræ Petit assigns a depth exceeding 4000 miles. On the other hand, Phillips considered 300 miles a probable amount. Neither estimate is primá facie entitled to much consideration.

Sir W. Herschel supposed that one of the hemispheres of the Sun is by its physical constitution less adapted to emit light and heat than the other, but the grounds of this conclusion are not known.

The study of the Sun has during the last few years taken a remarkable start, owing to the fact that by the aid of the spectroscope we have been enabled to obtain much new information about its physical constitution. This subject being, however, a physical rather than an astronomical one, and involving a great amount of chemical and optical detail, it cannot conveniently be discussed at length in a purely astronomical treatise, though something will be said concerning it later on in the portion of this work dedicated to spectroscopic matters.

CHAPTER II.

THE PLANETS.

Epitome of the motions of the Planets.-Characteristics common to them all.-

Kepler's laws.-- Elements of a Planet'x orbit.-Curious relation between the distances and the periods of the Planets.---The Ellipse.- Popular illustration of the extent of the Solar system.-Boile's law.-- Miscellaneous characteristics of the Planets.-Curious coincidences.-Conjunctions of the Planets.-Conjunetions recorded in History.- Different systems.---The Ptolemaic system.-The Egyptian system.The Copernican system.The Tychonic system.

A ROUND the Sun, as a centre, certain bodies called Planets a 11 revolve at greater or less distances b. They may be divided into two groups, (1) the “inferior” planets, or those whose orbits are within that of the Earth, namely Vulcan (?), Mercury, and Venus ; and (2) the “superior” planets, or those whose orbits are beyond that of the Earth, namely Mars, the Minor Planets, Jupiter, Saturn, Uranus, and Neptune.

If viewed from the Sun all the planets would appear to the spectator to revolve round that luminary in the order of the zodiacal signs; such, however, cannot be the case when the observation is made from one of their number itself in motion, and therefore to us on the Earth the planets appear to travel in a capricious manner; and, further, the inferior and superior planets differ the one class from the other in their visible movements.

The Inferior planets are never seen in those parts of the heavens which are in Opposition to the Sun; in other words,

a glavntos, a wanderer.
• The distances of the planets are

reckoned in all cases from the centre of
the Sun, and not from its surface.

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