3rd mag. as 2nd. 4. The symbol of the 5th mag. stars is a faint diminutive cross, almost invisible to the eye, and bearing no relative proportion to the 4th mag., which it was designed to succeed. 5. This same little cross is really the 8th mag. star on the larger Maps, and causes endless confusion to one who consults both the larger and smaller Maps at the same time. The uniformity ought to have been preserved by the usual four-petalled stars. 6. The 6th mag. stars are excluded altogether from the smaller Maps-" a very serious fault," for the 6th mag. stars are readily visible to a good eye; they serve to identify certain localities, and in many instances they serve as excellent pointers to invisible telescopic objects. 7. The great star-clusters, and many of the double stars, are indicated on those smaller Maps, not in the simple and beautiful manner found on the larger Maps and on Malby's Globe Atlas, but by a minified triangle, which is, to say the least, very unsuitable. 8. Flamsteed's Arabic numerals, which are considered very important helps in distinguishing the stars, are very generally omitted on these Maps-not for want of room, for blank space is everywhere predominant. In the instances where Flamsteed's numerals are supplied, there is no difference of type to distinguish them from Piazzi's numbers, as there is on the larger Maps. 10. The nebula of Messier's Catalogue, which embraces some of the finest objects in the heavens, are not characterised by an M. (as 8 M., 22 M.), to distinguish them from the nebulæ of Sir William and Sir John Herschel. The nebula of Sir W. Herschel are not distinguished by their classes, as I.H.IV., &c.; nor are they distinguished from those of Sir John Herschel, as in the larger Maps. 12. Several grand objects, visible to the naked eye, as 8 Messier (see Webb's Catalogue, p. 231), are omitted altogether; so that I conclude Mr. Dawes has not discharged his duty as editor very efficiently. His work on those Maps was, if I am rightly informed, the only effort he ever made for the benefit of tyros in astronomical observations.-Faithfully yours,

9.

W. A. DARBY, M.A.

II.

The Astronomical Register.

No. 36.

DECEMBER.

1865.

RECENT OBSERVATIONS ON THE SUN BY SECCHI.

THE following interesting memoir has been translated from No. 1553 of the Astronomische Nachrichten expressly for the Astronomical Register:

"Astronomers will no doubt have observed pretty generally the very interesting solar spot which has lately been visible, but possibly many may not have enjoyed the opportunities we have for its lengthened examination. Permit me, therefore, to lay before you some account of what we have seen, reserving for a future publication the fullest possible remarks. There will be no disadvantage in this course, for the solar question has arrived at that stage that without detailed observations, we cannot proceed further.

"I think that I assisted, so to speak, at the first formation of the spot. On July 28, nothing was visible in its place; on the 29th, there were only two almost imperceptible points. On July 30, I found at 10h. that the spot had become enlarged to the immense diameter of 76", and that it occupied the place of one of the two points seen on the preceding day, whilst the other point remained in statu quo.

"Its figure was almost circular, and it was made up of a circle of very small spots, which, observed in our great refractor, with a diagonal eye-piece, presented the appearance of being whirlwinds (tourbillons) of very irregular shape. I made immediately

a drawing in the half-hour between 11:30 and 120, which portrays the spot with all the accuracy practicable in dealing with an innumerable multitude of details, varying inter se from minute to minute. At 3h. p.m. I made another sketch, but great changes had taken place. In the first drawing there were traces of tourbillons, which developed themselves in separating from one another. An idea of the extent of the transmutations will be gained by comparing photographs 1 and 2. It will be seen that the principal features of the photograph of July 31 are but the development of those of the preceding day, and that the chief alteration in aspect was the formation of large isolated tourbillons. The remaining changes, even to the time of the final disappearance of the limb, consisted in a more exact determination and separation of the great centres of movement or of the nuclei, the smaller being absorbed by the larger, and the larger spreading themselves more and more. Thus the central part of the circle of the first day, which was granulated like the rest of the Sun's surface, had disappeared (having been absorbed by the spots belonging to the margin) and had given place to a great extension of penumbra, upon which a multitude of long white bodies, resembling cotton filaments in a state of tension, were projected (se dessinaient not projetaient).

In seeking for some terrestrial analogy with which to compare the vagaries of this spot, I could find nothing better than the great agitation produced in a mass of running water, for example in a river, of which the final result is to produce a number of independent whirlpools which are carried forwards by the current itself, with directions somewhat parallel to each other. The length of the group on Aug. 1 had reached 141", the breadth in latitude remained pretty stationary, that is to say it was 74". The enormous longitudinal expansion which the spot underwent appears to indicate a systematic motion of translation, which found itself backed up on one side by an impulse received at an early stage of the spot's existence, whilst on the other side it took place in a contrary direction. It was impossible to take micrometrical measurements of this object; we could not do more than make diagrams on a large scale, which I hope soon to publish.

"I now come to the most important observation. I have already remarked that in its advance the spot had suffered a great pro

longation of its interior penumbra, and that in that penumbra there existed well marked nuclei, surrounded by facules or more luminous matter. I watched then with great keenness the disappearance of the spot at the sun's limb, to see what phenomena manifested themselves. The first part of the spot arrived at the limb on Aug. 5, at 6h. p.m., but the agitated state of the atmosphere did not permit me to observe satisfactorily the precise appearances. On the following morning the second part of the spot reached the edge of the sun, and I had the good fortune to see what is now to be described under very favourable circum

stances.

"On the preceding day (Aug. 5) the spot had a nucleus which in the morning [of the 6th] was very near the limb. This nucleus showed itself as a very black mark, separated by a very luminous thread from the sun's edge, which thread produced a prominence on the edge; right and left of this prominence was a very obvious depression. At some distance from the nucleus, reckoned along the margin of the sun, there was a luminous region, and beyond that the limb raised itself again, and continued in undulations as far as the exterior limit of the spot, beyond which the solar limb recovered its usual regular curvature. I was absorbed in this spectacle, when I received a visit from M. Tacchini, the Astronomer of Palermo, a man highly skilled in solar observation, and without saying anything to him I begged him to examine the spot, and to pourtray on paper what he had seen. M. Tacchini made a sketch exactly representing what I myself saw.

"At 10h. 32m., Rome M.T., the luminous border which surrounded the nucleus was still visible as an attenuated line, but with very palpable knots and irregularities. It was impossible not to liken these appearances to those which one sees on the border of the Moon when near its full. A photograph of the Moon in this condition, which reached me that very day from Mr. Rutherford, of New York, convinced us that the two phenomena were analogous. For greater security in making an observation of such importance, we took a view of the border by projection, and we estimated the irregularities to be between and second of arc.

"From these facts it seems to me established that the faculæ are prominences and the penumbræ depressions on the Sun's surface. If these depressions are so seldom seen, it is be

cause the spots are ordinarily very small as compared with the Sun, and so it happens that the projecting edge of the interior part, when the spot is near the limb itself, hides the penumbra and the cavity, as is the case in fact with the Moon. But when the depression has a vast area, as in this instance, the projecting edge cannot encroach sufficiently to hide all the depressions of the penumbra. It is needless to insist on the importance of this observation, which decides for ever this doubtful point.

"In the drawings, which I send you, you will find pourtrayed the constitution of the penumbræ, as seen by me. It is true that the Willow-leaves* are not perceptible; let me remark, they are best arranged in that part of the penumbra which is on the exterior side of the great mass of spots; they are very confused and interlaced on the interior side. They appear even stretched and muddled together in the central part, besides which it is impossible to admit that these elongated bodies are at all solid. I saw them both spring up and melt away under my eyes on that occasion. I could not form any other idea than that they were clouds, and that the material is driven by a force of aspiration towards the centre of the nucleus.

"I shall conclude my present remarks by describing a curious result, which I obtained from micrometrical measures made on a fine circular spot visible on July 20. I took measures of distance between the spot and the Sun's limit with the greatest possible care, and under very favourable atmospheric conditions.

"The result was as if the arc of rotation became successively less, as fast and as soon as the spot approached the limb. Thus I obtained for 24 hour intervals (evening observations):

Between July 17 and 18, Rotation of spot 14 40

It may here with propriety be stated that Secchi seems to have become a convert to the Willow-leaf theory. At least in another letter in A. N. 1553, dated July 14, 1865, he says, speaking of what he saw and wrote about in 1853: "Seulement la dénomination de feuilles de saule paraît exprimer mieux certains détails, mais au fond les choses sont les mêmes."