THE duration of individual spots is a matter associated with extremes both ways. Some remain visible for several months, others scarcely for as many minutes; but a few days or weeks will commonly be found the usual extent of permanency. Some are formed and vanish during the period of a single transit (rather more than 12 days), others remain during several successive transits; for it will be readily understood that the Sun, being endued with an axial rotation, and the spots being fixed (or nearly so) on the Sun's surface, it will not be possible for any one spot to remain in sight consecutively for longer than the semi-duration of the Sun's rotation. When observed for any length of time, a spot will first be noticed on the eastern limb, disappearing in little less than a fortnight on the western limb; after an interval of nearly another fortnight, the spot, if still in existence, will reappear on the eastern side, and in like manner traverse the disc as before. This phenomenon necessarily can only be accounted for on the supposition that the Sun rotates on its axis; and observations specially conducted with that object in view will give the period of this rotation, which Laugier ixed at 25d. 8h. 10m.—a result fairly of accord with Bianchini's, of 25d. 7h. 48m., deduced in 1817, when the difficulties attending the observations due to the ever-varying forms of the spots are taken into consideration.

On February 19, 18co, Sir W. Herschel states that he was watching a group, but that on looking away even for a moment, it could not be found again.* The same observer followed a spot, in 1779, for six months; and, in 1840 and 1841, Schwabe observed one and the same group to return eighteen times, though not consecutively.† In July, August, and September 1859, a Phil. Trans, vol. xci. p. 293. 1801. + Ast. Nach., No. 418.

large group was followed through several apparitions, and another very noticeable instance of the kind occurred in the autumn of 1865. Similar cases are by no means very rare. It has been surmised, and Sir J. Herschel thinks "with considerable apparent probability," that some spots at least are generated again and again, at distant intervals of time, over the same identical points of the Sun's body. There does not appear to be much evidence to bear out this hypothesis,* and the fact now recognised, that proper motion exists with some of them, is of course directly counter to it. The Rev. T. W. Webb says: "Fritsch has stated that he has seen one stand nearly still for three days; and Lowe has even witnessed retrogradation-assertions which may involve a suspicion of mistake. Schröter and others have ascribed a more moderate locomotion. This was micrometically established by Challis in 1857; and Carrington has recently made known his very interesting discovery, that there appear to be currents in the photosphere drifting the equatorial spots forward in comparison with those having considerable latitude, with lateral deviations of smaller amount. With these shifting landmarks, it is not surprising that the Sun's period of rotation is still doubtful, though Laugier's value cannot be far wrong. Relative displacement in groups would be an interesting study, requiring neither micrometer nor clock, only careful drawing. Several observers have found that spots near the limb require a different focus from those near the centre; this is, no doubt, as Dawes says, an optical deception."†

The varying position of the Earth with reference to the Sun, combined with the inclination of the axis of the latter to the plane of the ecliptic (amounting to 82° 41' according to De Lambre; to 83° 9' according to Petersen ‡), gives rise to the fact that at no period of the year do the spots appear to traverse the Sun's disc exactly in the same way. In June and December, the Earth is in the line of nodes of the spots-or, in other words, its longitude, as seen from the Sun, corresponds nearly with the points of intersection of the solar equator and the ecliptic-and the paths of the spots are then inclined straight lines. In March, the South pole is most turned towards us, and the tracks are concave towards the South; in September, the conditions are precisely reversed in every respect; at intermediate periods, the paths are both inclined and curved at the same time.

Individual spots also possess many personal peculiarities. * Sir John appears now to recant. In his able Memoir in the Quart. Journ. Sc. for April 1864, already cited, he says exactly the opposite. Celestial Objects, p. 28.

The longitude of the ascending node for 1850 is 73° 40'; so that the North pole of the sun's axis points nearly to Draconis, and the South one to a Plutei.

Dawes observed one on Jan. 17, 1852, which, by the 23rd of that
month, had rotated in its own plane through 90°. Birt believes
the same thing happened with a spot he scrutinised in February
and March 1859. * Schwabe has seen occasionally spots of a
reddish-brown colour, under circumstances of contrast precluding
the possibility of deception; on one occasion three telescopes and
several bystanders certified to this. In 1856, Capocci perceived
a violet haze issuing from each side of the bright central streak of
a great double nucleus; and during the eclipse of March 15, 1858,
Secchi saw a rose-coloured promontory in a spot visible to the
naked eye.
Schwabe describes the penumbræ as made up of a
multitude of black dots, usually radiating in straight lines from
the nucleus, which radiations superior optical power enables
Secchi to define to be alternate streaks of the bright light of the
disc and the dark ground of the nucleus.†

In

Some of these spots are of prodigious size, and are therefore visible to the naked eye. A few recent instances are here given. A spot measured by Pastorff on May 24, 1828, was computed to have an area about four times the entire surface of the earth. June 1843, Schwabe observed one 2'47", or 74,816 miles in diameter. It was seen for an entire week without the aid of a telescope. On March 15, 1858, the day of the celebrated eclipse, a spot, having a breadth from west to east of 4', or 107,520 miles, attracted considerable attention. On September 30, in the same year, one having a breadth from west to east of 5'21", or 143,808 miles, was observed.§ On January 26, 1859, and during August, large spots were seen; one of the latter measured nearly 58,000 miles, according to Newall, who saw it distinctly as a notch on the edge of the Sun's disc, the like of which he only saw once before-namely, on March 25, 1850.||

"The origin of a spot, when it can be observed, is usually traceable to some of those minute pores or dots which stipple the Sun's surface, and which begin to increase, to assume an umbral blackness, and acquire a visible and, at first, very irregular and changeable shape. It is not till it has attained some measurable size that a penumbra begins to be formed—a circumstance strongly favouring the origination of the spot in a disturbance from below, upward; vice versa, as the spots decay they become bridged across, the umbræ divide, diminish in size, and close up, leaving the penumbræ, which, by degrees, also contract and disappear. The evanescence of a spot is usually more gradual than its forma* Month. Not. R.A.S. vol. xix. p. 182.

The preceding facts are given on the authority of Webb, op. cit. p. 22. He gives no references, so I am unable to verify them.

Month. Not. R.A.S. vol. xviii. p. 193 and elsewhere.

Ast. Nach., No. 1182.

Letter in the Times, Aug. 27, 1859.

tion. According to Professor Peters and Mr. Carrington, neighbouring groups of spots show a tendency to recede from one another."*

The most indifferent observer can hardly fail to be struck with the rapidity of the changes which take place in solar spots. Dr. Wollaston says:-"Once I saw, with a 12-inch reflector, a spot burst to pieces while I was looking at it. I could not expect such an event, and therefore cannot be certain of the exact particulars; but the appearance, as it struck me at the time, was like that of a piece of ice when dashed on a frozen pond, which breaks in pieces, and slides on the surface in various directions. I was then a very young astronomer, but I think I may be sure of the fact." Their immense number is likewise very noticeable. On April 26, 1846, Schmidt of Bonn counted upwards of 200 single spots and points in one of the large groups then visible, and 180 in another cluster in August 1845. On August 23, 1861, I counted 70 distinct spots with a telescope of only 3 inches aperture, charged with a power of 21. Schwabe finds that the western members of a group disappear first, and that at the eastern end fresh ones are apt to form, where also the junior members are most numerous; that the small points are usually arranged in pairs (much after the appearance of the dumb-bell nebula), and that, when near the edge of the Sun, the penumbræ are much brighter on the side next the limb. Sir J. Herschel has often noted the penumbræ to be least defined on the preceding side; and Capocci found the principal spot of a group usually the leader. The same observer believed the nuclei to be better defined in their increase than when going through the opposite stage.

Attention has now to be directed to one of the most curious and interesting discoveries of modern astronomy-the periodicity of the solar spots. The renowned Schwabe, of Dessau, is the hero of this. He shall be introduced to the reader by a repetition of the words of the late Radcliffe Observer, when President of the Royal Astronomical Society, and speaking on the award to him of the Society's Gold Medal in 1857-"What the Council wish most emphatically to express is their admiration of the indomitable zeal and untiring energy which he has displayed in bringing that research to a successful issue. Twelve years, as I have said, he spent to satisfy himself; six more years to satisfy, and still thirteen more to convince, mankind. For thirty years, never has the Sun exhibited his disc above the horizon of Dessau without being confronted by Schwabe's imperturbable telescope, and that appears to have happened, on an average, about 300 days a year. So, supposing that he observed but once a day, he has made 9,000 *Sir J. Herschel, in Quart. Journ. Sc. April 1864, p. 225.

observations, in the course of which he discovered 4,700 groups. This is, I believe, an instance of devoted persistence (if the word were not equivocal, I should say pertinacity) unsurpassed in the annals of astronomy. The energy of one man has revealed a phenomenon that had eluded even the suspicion of astronomers for zoo years."

At

The gist of this discovery is expressible in a few words-that the spots are subject to a periodical variation in prevalence to the amount of about 10 years, during which time they pass habitually between the extremes of a maximum and minimum number. the epoch of minimum, on many days absolutely no spots are to be seen; and it has been pointed out by Sir J. Herschel, that the epoch of maximum does not appear to fall exactly in the middle year of the period, but rather earlier.†

(To be continued.)

THE PRESIDENT'S SOIREE.

THIS, which we hope may now become an annual institution, took place at Willis's Rooms on January 17. The company, a large and distinguished one, began to assemble at about 9 o'clock, and were received at the entrance of the large room by the President, Mrs. De la Rue, and the Misses De la Rue. Ices and light refreshments were served early in the evening, and a more substantial repast in the form of a champagne supper at about 10.30. The Royal Artillery band was in attendance and played a variety of pieces, interspersed with some part-songs. It is impossible for us to set out at length all the objects of interest exhibited: we shall therefore select what appear to us to have been the more important. Perhaps Mr. Glaisher's balloon car, as fitted up for his meteorological observations, attracted the most general notice. To see this and all its complicated accessories set out in full working order, made it a somewhat difficult matter to understand how one man could accumulate, in the comparatively short space of a few hours, such a vast mass of results as does Mr. Glaisher. Hung around the walls of the room were a large number of drawings, engravings, and photographs of lunar craters, sun spots, planets, &c.

The lunar sketches (by Mr. Nasmyth) were very beautiful specimens of art; chalk drawing, we believe. Mr. Williams exhibited a lithographic copy of a rubbing of a brass prepared by a very simple but useful process. In place of ordinary rubbing materials, transfer paper and a lithographic compound were employed, and in this way the rubbing can at once be transferred to *Month. Not. R.A.S. vol. xvii. p. 129.

Outlines of Ast. p. 253.