My object in the February number is not so much intended to give a representation of the moon's motion as it is (so to speak) to analyse the motion of a body which has caused so much dispute. I think this illustration bears upon M. Perigal's argument, and I think it will show Nauticus that the very fact of his fastening the body on his rod ensures rotation of the body when the rod is bent into a curve. I am, dear sir, yours truly,

W. M. [Any further correspondence on the moon's rotation can be admitted only on the conditions given in paragraph 7 in our last number.—ED.]

NOTICE OF SOME ERRATA IN THE REV. W. A. DARBY'S "ASTRONOMICAL OBSERVER."

TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir,-It will, no doubt, be readily granted that in a hand-book or guide-book, whatever may be its subject, the accuracy of the information it professes to give is a matter of the first importance. However large may be the range of the information, if its correctness cannot be relied on, it will probably only mislead in a number of instances, the greater in proportion to its extent. It is with great regret that I have found these remarks applicable to the Rev. W. A. Darby's Astronomical Observer or Handbook to the Observatory. The object and plan of the body of the work is excellent, being, in fact, an extension of the sidereal portion of the Rev. T. W. Webb's Celestial Objects for Common Telescopes: and if it were but comparable to that valuable little volume, in the important point of accuracy, it would be, in that department, proportionately more useful. Unfortunately, a cursory inspection of the book convinced me that it was so far from possessing that qualification, that it would be desirable to point out a few of the errors I had met with, both for the benefit of the tyros in observation, for whom it is principally designed, and also to attract attention to the importance of a careful revision of the whole work in case of a second edition being called for.

Confining my notices to errors of an astronomical character, I have selected the following:

Introduction, p. xxiii.-The example of converting sidereal time at Manchester into Greenwich mean solar time is erroneous in its method, as may be proved by re-converting the G. M. T. found, viz. : 7h. 26m. 11'50s. into Manchester S. T., according to the method in Example II.; and the error would have been greater, but for the counterbalancing error in the reduction of the given sidereal time into the equivalent mean time.

Introduction, p. xxv.-In the list of test objects the epoch of the places is 1865; but the epoch of the distances given in the last column is very different; and consequently, in several of the binary stars included in this and in the list on the next page, the distances given are very erroneous. In Admiral Smyth's Cycle, from which the

list is taken, the epoch of the places is 1840. The omission of the epoch to which both the positions and distances of the binary stars belong is a serious fault, which, with few exceptions, pervades the whole book.

P. 5.-Antinous. 185 p. xix, and 186 p. xix. Mr. Darby seems to be quite unaware of the mistakes which have been committed about these objects, and which I noticed in a communication published in the Monthly Notices R.A.S. for Nov. 1862. The description of these objects is almost entirely erroneous.

P. 6,- Aquarii is not "widening," but has been becoming closer for many years past. The omission of dates produces confusion. For "3"5, Bishop; 3"9, Dawes," read "3"9, Dawes in 1832-3; 35, Dawes in 1841-3.' 19 In 1818 it was easily separated with my 16-inch O.G., as is shown by a diagram of it in my astronomical note-book of that year.

For "dist. 1"5," read "dist. 95"+"!

For "b 15," read "b 8."

P. 8.- Aquila. P. 14.-179 p. 1. P. 18.-μ Bootis. The omission of the dates makes confusion in the distances, which were, in 1833, 1"17 (Struve and Dawes); and in 1840-2, 085 (Dawes). A year or two ago it had decreased to o"5+. P. 20.- Cancri. For "dist. a-b, 1"3," read "dist. a-b, ɔ"65 (1864)." At the top of p. 21, Mr. Darby says, with reference to the same triple star: " has more than completed a whole revolution round 6 since first observed by H." For "c," read "b"; and for "b," read "

a,"

P. 24.-Sirius. Goldschmidt's refractor, aperture 46 (Paris) lines, is said to "3 English inches." The correct equivalent is about 4'1 inches.

P. 26.-Procyon. Admiral Smyth's lost star following Procyon at a distance of 145" is confounded with the 8 mag. star whose distance is about 330"! It is this which was discovered by Mr. F. Bird to be close double. Smyth's star has not yet reappeared. P. 30.-55 Cassiop. For "a-b, "8," read "a-b, 18'8” (time). Both the A.R. and Dec. of this star are given erroneously. For "zh. 11m. 318." read "2h. 3m. 56s."; and for "65° 23′ 24′′,” read "65° 53′ 22′′."

P. 37.-1 Corona. The dist. of the components is now about "o, instead of o" 5. For " (iota) Bootis," read "i (italic) Bootis." It is 44 (i) Bootis which is referred to.

P. 37. Corona. For "a-b, 1"8," read "a-b, 3"o." The dist. given is that of 20 or 25 years ago.

P. 49.- Gemin. "Dist. 10"3 (7" Bishop).' Where is this distance registered which is quoted as "Bishop's?" I know nothing of it.

P. 50.- Herculis. For "dist. "'z," read "the star is now single to all telescopes of moderate aperture, as 8 or 10 inches.

P. 60. 215 (Libra). This star is not double at all. The whole description is a mistake; as is also the remark that the R.A. is erroneously given in Mr. Bishop's volume. Mr. Darby has confounded it with 51 Libra (Flamsteed), respecting which Admiral Smyth's just remark is quoted, that this star is really & Scorpii. It is surely strange that Flamsteed's obvious mistake should have been adopted at all.

Argelander, with his usual accuracy, has restored the star to Scorpio, and omitted 51 Libra. This he did in 1840.

P. 60.-51 (Libra) = Scorpii. For "ab, ":2," read "a-b, o"6." The star has lately been nearly single; but is now beginning to separate again.

P. 72, 1. 5. The "W. Lassell, jun." and "Mr. Lassell (senior)" here referred to are one and the same individual-at present in Malta. He independently discovered the sixth star in the trapezium, with his Newtonian of 9 in. aperture, in 1841-2; but it had been first discovered, I believe, by Sir J. Herschel, with Sir James South's 11-in. refractor.

P. 80. -209 p. 1. Instead of "dist. 15," it is now scarcely o".7. P. 90.-7 Tauri. “H. not perceiving c." For "c" read

"b."

It is to be regretted that, under the constellation Orion, the beautiful double stars n (eta) and 42 (c′) are entirely omitted, as also some other delicate objects which are comprised in my list of new double stars, published in the M. N. of the R. A. S. for March 1864. n is evidently binary, and it is important that it should be well observed; and others in the same prolific vicinity may turn out to be so. I remain, Sir, yours faithfully, Hopefield Observatory, Haddenham, Thame: W. R. DAWES. April 1865.

PERIODIC COMETS.

As a very interesting comet of this class is now approaching its perihelion (Encke's), I propose to say something on the history of the short period comets, in the belief that a concise statement of the kind will not be unacceptable to the readers of the Astronomical Register.

G. F. C.

No. 1 is by far the most interesting comet in the list, and I shall therefore renew its history somewhat in detail.

On January 17, 1786, Méchain, at Paris, discovered a small telescopic comet near the star 3 in the constellation Aquarius. On the following day he announced his discovery to Messier, who, from some cause or other, did not see it till the 19th, on which night it was also observed by Cassini and the original discoverer. It was tolerably large and well defined, and had a bright nucleus, but no tail.

On November 7, 1795, Miss Caroline Herschel, sister of the celebrated Sir W. Herschel, discovered a small comet, about 5' in diameter, without a nucleus, but yet having a slight central condensation of light. Olbers observed it on November 21, when it was too faint to allow of the field being illuminated, and he was obliged to compare it with stars in the same parallel, by noting the times of transit across the field of view. It was round, badly defined, and about 3' in diameter. The orbit greatly perplexed the calculator, and Prosperin declared that no parabola would satisfy the observations.

On October 19, 1805, Thulis, at Marseilles, discovered a small comet, which was faintly visible to the naked eye. Huth states that on the 20th it was very bright in the centre, though without a nucleus, and 4′ or 5′ in diameter. On November 1, the same observer saw a tail 3o long. Several parabolic orbits were calculated, and one elliptic one by Encke, to which a period of 12 127 years was assigned.

On November 26, 1818, the indefatigable Pons, of Marseilles, discovered a telescopic comet in Pegasus, which was very small and illdefined. As it remained visible for nearly 7 weeks, or till January 12, 1819, a rather long series of observations were obtained, and, finding that under no circumstances whatever would a parabolic orbit fairly represent them, Encke determined rigorously to investigate the elements according to the method of Gauss, then but little practised. Having done this, he found that the true form of the orbit was an ellipse, with a period of about 3 years. On looking over a catalogue of all the comets then known, he was struck with the similarity which the elements obtained by him bore to those of the comets of 1786 (i), 1795, and 1805, and he was strongly impressed with the idea that the one then under investigation was identical with those comets, more particularly as, in the assumption of a 3 year period, it might be expected to have been in perihelion at about those epochs. This question could only be settled by calculating backwards the effects of planetary perturbation, which Encke did by an extraordinary effort in 6 weeks. He was accordingly able to assure himself of the identity of the comet of 1818 with the 8 above-mentioned ones, and also that between 1786 and 1818 it had passed through perihelion 7 times without being seen.

Encke then turned his attention to its next return, and he announced that the comet would arrive at perihelion on May 24, 1822, after being retarded about 9 days by the influence of the planet Jupiter.

"So completely were these calculations fulfilled, that astronomers universally attached the name of 'Encke' to the comet of 1819, not only as an acknowledgment of his diligence and success in the performance of some of the most intricate and laborious computations that occur in practical astronomy, but also to mark the epoch of the first detection of a comet of short period- one of no ordinary importance in this department of science.

It unfortunately happened that at its return in 1822 the position of the comet in the heavens was such as to render it invisible in the northern hemisphere. It was therefore systematically watched by only one observer, M. Rümker, who discovered it on June 2, at the private observatory of Sir T. M. Brisbane, at Paramatta, and he was only able to follow it for 3 weeks. Rümker's observations were, however, so

far valuable that besides showing that the comet actually did come back, they furnished Professor Encke with the means of predicting with greater certainty its next return, which he found would occur on September 16, 1825. G. F. C.

To be continued.

NOTES AND GLEANINGS.

SPECTRA OF THE STARS AND NEBULE.-A lecture on this subject will be delivered at the Royal Institution, Albemarle Street, on Friday, May 19, by Mr. Huggins, in which he will state the result of his recent labours.

DOUBLE STARS.-A useful series of double star measurements, performed at Leipzig during 1864, by M. Englemann, will be found in Astronomische Nachrichten, 1518; March 19, 1865. Nearly 50 stars are dealt with, all of them from Struve's catalogue.

THE BINARY STAR & CYGNI.-M. Behrman, of Göttingen, has devoted a good deal of attention to the investigation of the orbit of this star, and he finds its period to be 280 years, a value differing materially from Hind's, which is only 178 years.

AMERICA has just been deprived of another of its most talented astronomical savants, Captain J. M. Gilliss, Director of the Naval Observatory at Washington, who died on February 9th.

OCCULTATIONS OBSERVED AT TEIGNMOUTH. By J. G. Walker,

Esq.

Latitude, 50° 32' 48" 5

A Cancri, March 8, 1865.

Star bright to the last: power used, 175.

Longitude, W., 13m. 588.9.

Disappearance

6h. 57m. 178.

a Libræ, April 12.

Disappearance

Reappearance

Moon's limb tremulous at immersion: star bright to the last.

We extract from the "Monthly Notices" the following observations of the former occultation by Capt. W. Noble, of Maresfield :

A' Cancri, March 8.

Reappearance, 7h. 43m. 58'1 local sidereal time.
= 8 36 5187 local mean time,
Telescope, 4'2 aperture. Power, 135.

CORRESPONDENTS' QUERIES AND REPLIES. Can you or any of the readers of your valuable Register say when will all the moons of Jupiter be invisible again?

H. J.

WATER ACTION ON THE MOON.-If F. B. will turn his telescope on the remarkable spot Fracastorius, just south of the Mare Nectaris, described as a "bay" by Beer and Mädler, he will see an instance of a class of lunar forms difficult of explanation, inasmuch as while the