bution of time signals and the rating of chronometers could be attended to, in addition to the preparations for removal. Mr. Fletcher, at Tarn Bank, is employing his equatorial refractor, of 9 inches aperture, in re-observing the Bedford Catalogue before publishing his new edition of the Celestial Cycle of Admiral Smyth. At Cranford Mr. De la Rue has obtained more lunar photographs, especially during the Eclipse of October 4, which combine stereographically with those of February 1858. Biela's Comet has been unsuccessfully swept for, and a new Observatory, to contain a fine Achromatic Equatorial, by Merz, has been erected. Mr. De la Rue has also commissioned Messrs. Cooke to construct an object-glass of 13 inches diameter, having the visual and actinic foci coincident, for the purpose of photographing the Sun on a scale of 3 feet in diameter. Mr. Huggins has continued his spectrum analysis of the heavenly bodies at Tulse Hill, and obtained the remarkable results on Comet 1, 1866, mentioned at the last meeting of the Society; he has also analysed the light of about 40 additional nebulæ and clusters, and made an attempt to determine approximatively the intrinsic brightness of three of the gaseous nebulæ. It is probable that these bodies consist of continuous masses of material. In the telescope they present surfaces subtending a considerable angle. As long as a distant object is of sensible size in the telescope, its original brightness remains unaltered. By a suitable method of observation the intensities of these nebulæ have been obtained in terms of the light of a sperm candle burning at the rate of 158 grains per hour. The Light in Nebula 4628 1 H. IV. Annular Nebula in Lyra Dumb-bell Nebula = 6032 th 19604th = = 150th part of that of the Candle. This estimation, in each case, refers to the brightest part of the nebula. Nebula 4628 gives a spectrum of three lines, and also a faint continuous spectrum. The nebula in Lyra and the Dumb-bell nebula give one bright line only. These values are only approximative, and are too small by the unknown corrections for the possible power of extinction of space and for the absorptive power of the Earth's atmosphere. Similar observations made at intervals of time may show whether the brightness of these strange bodies is undergoing increase, diminution, or periodic variation. The labours of Lord Wrottesley on double stars; Mr. Birt, on lunar delineation; and Professor Grant, at the Glasgow Observatory, were also alluded to. Progress of Astronomy. It is extremely difficult, if not impossible, to estimate correctly the amount of actual progress made in a science such as Astronomy during any single year. Not seldom the real work lies for the moment either not wholly matured, or by its very nature is to be stored away as seed for the harvest of future years. All that can be done in this direction is to give, so far as may be, a faithful record of what has actually come to the surface, and has already found a place in the scattered annals of learned Societies. Under the head of Cometary Astronomy, Mr. Huggins's observations, already mentioned, were again referred to; also the fact that, despite the excellent ephemerides provided by Mr. Hind, Biela's Comet had not yet been found. Mr. Hoek's Papers and Cometic Hypothesis were also noticed. Mr. Hoek considers that he has advanced mathematical grounds whereon to found the probability that every star is associated with a cometary system of its own; but that, owing to the attraction of planetary or other cosmical matter, these bodies continually leave their proper primaries and revolve, either permanently in ellipses, or temporarily in parabolas or hyperbolas, round other Suns. In astronomical photography-besides the results obtained by Mr. De la Rue and the Kew Observatory-Mr. Brothers, of Manchester, has taken pictures of the late Lunar Eclipse, and very fine photographs of the Moon have been received from America, one of 20 inches, and the other 24 inches in diameter-the first produced by Dr. Draper, with a silvered glass mirror; the second by Mr. Rutherford, with an 11-inch object-glass. The latter is admitted by Mr. De la Rue to be the finest lunar picture yet obtained. On the subject of the Acceleration of the Moon's mean motion, M. Delaunay thinks that he can satisfactorily account for that outstanding part of the acceleration which at present appears not to be accounted for by planetary disturbance. On the hypothesis that the disturbing forces of the Sun and Moon act on the lagging protuberance of the great tidal wave, he considers that the amount of this action is quite sufficient to produce a slight but continuous augmentation in the time of the Earth's rotation on its axis, sufficient to account for the outstanding 6" of the Moon's acceleration. Whether this be the case or not, this remarkable conclusion of M. Delaunay can scarcely fail to give an additional impetus to the re-consideration of the more difficult and obscurer parts of the Lunar Theory. In connexion, again, with the alteration in the eccentricity of the Earth's orbit, so intimately related to the former question raised by M. Delaunay, stand some remarkable speculations very recently raised or renewed by Mr. Croll relative to the alleged effects of climatal heat or cold produced in the course of many thousands of years by the variation in the aphelion and perihelion distances of the Sun from the Earth. Geologists give us ample evidence of at least one glacial period, and they are now beginning to observe indications of a succession of these periods of refrigeration, separated from each other by very long intervals of time. No doubt during the course of these immensely separated epochs there have been cycles of change in the eccentricity of the Earth's orbit, and it will be the province of mathematicians and physicists, possessing competent skill, to determine how far such a cause is sufficient to account for the effects produced. Meteoric Astronomy is gradually being brought within the domains of known physical law. Regular observations, whenever the state of the atmosphere admits, are made at the Royal Observatory, at all the epochs of meteoric activity which have as yet been established or suspected. The well-known periodic starshower of November was very diligently and successfully observed at Greenwich under Mr. Glaisher, at Cambridge under Prof. Adams, and at Hawkhurst by Mr. Alexander Herschel. Some important calculations have been made from occultations of Stars by the Moon as observed at Greenwich and Cambridge, with a view of ascertaining the difference between the semidiameters of the bright and the dark Moon. The result is that the Greenwich instruments give a telescopic semi-diameter too large by about 2". Mr. Airy considers it probable that the whole of these 2" are due to irradiation; and he remarks that, even if the whole of it were supposed to be caused by a lunar atmosphere, its attenuation must be so great that it would probably be discoverable by no other mode of observation. The accurate measurements of Mr. De la Rue's photographs of the great Solar Eclipse of 1860 give precisely the same results; a circumstance which leads to the hope that photography may ultimately become a valuable auxiliary in even micrometrical observations. Mr. J. Gurney Barclay has recently printed a volume of observations made at his private Observatory, Leyton, during the years 1862-64. Dr. Brünnow, lately of the Observatory Ann Arbor, U.S.A., has recently been appointed to succeed the late Sir W. Rowan Hamilton at Dublin, as Astronomer Royal for Ireland. Presentation of the Gold Medal to Mr. Adams— Mr. De la Rue commenced by observing that "It may be truly said of the theory of Gravitation that while, on the one hand, its fundamental laws are easily apprehended by the human intellect, on the other, the results which the application of this theory produces are of so intricate a nature, especially when the mutual action of more than two bodies is concerned, that it requires the exercise of the highest faculties of minds specially cultivated, to trace out all the consequences which arise out of the mutual attractions of the particles and masses of matter which constitute the universe. "No application of the laws of gravitation has given more trouble to mathematicians than the development of the lunar theory. Frequently, indeed, in the history of physical astronomy grave doubts have been entertained respecting the possibility of accounting, by means of the grand and simple law discovered by Newton, for the perplexing inequalities to which the Moon's motion is subject, and which observation has proved to exist; and even the results of observations themselves have been called into question when the mathematician failed to account for the phenomena they disclose. "The medal has been awarded to Professor Adams for his investigations in respect of the Lunar Parallax and the Secular Acceleration of the Moon's mean motion." The President proposed first to take into consideration the last named of those works, which was communicated to the Royal Society so far back as June 1853, and published in the Philosophical Transactions, vol. cxliii. part iii. page 397. It is entitled, "On the Secular Variation of the Moon's Mean Motion." No contribution to astronomical physics has given rise to more discussion in modern times than the investigation of this problem by Professor Adams. A summary of the history of this famous problem is given with remarkable clearness by M. Delaunay in the Additions à la Connaissance des Temps for 1864. "Let me recall to your recollection that in dealing with the perturbations caused by the Sun in the Moon's motion, we have to consider only the differences between the action of the Sun on the Moon and on the Earth. If both bodies were attracted equally and in parallel directions, then their relative motions would not be disturbed, and would be in harmony with the laws of elliptical motion, as derived from the force of gravitation. But the distance of the Sun from the Moon and Earth, although great, is not so large but that the Moon is, in the course of her orbital motion round the Earth, sensibly nearer to the Sun at one time than at another; moreover, the angles which are formed by the line joining the Sun and Moon's centres with the line joining the Earth and Sun's centres, are sensible, though small. It results therefrom that the attractions of the Sun on the Moon and on the Earth are generally unequal, and act in different directions; and thus arises a disturbing action of the Sun, causing sometimes an increase, sometimes a decrease of the Moon's gravitation towards the Earth; sometimes a retardation and sometimes an acceleration of the areal velocity; also a change in the position of the modes. and in the position of the Moon's perigee; indeed, producing such a perplexity of inequalities that they can only be traced to their final result by discussing the effects of the disturbing forces with all the minuteness which the resources of analysis permit of. It is in the investigation of these various perturbations that consists the famous problem of three bodies, the rigorous solution of which, says Laplace, surpasses the resources of analysis; but that it fortunately admits of being resolved by approximation, in consequence of certain favourable conditions in the problem, for example, the proximity of the Moon to the Earth in relation to her distance from the Sun. He "The secular acceleration of the Moon's mean motion was discovered by comparing records of ancient eclipses with observation long before theory could account for it. Halley was the first who suspected its existence; he found, on computing the Moon's place for the epochs of certain ancient eclipses recorded by Ptolemy, by means of her mean motion computed from modern observation, that the result was such as to indicate a position in her orbit less advanced than her recorded place, and the time of an eclipse so calculated was later than that actually recorded. inferred therefore that the angular motion of the Moon must have been accelerated since the earliest astronomical records. Halley first alluded to this phenomenon in 1693, and nearly fifty years elapsed before this suspicion of his was confirmed. Dunthorne in 1749 communicated a paper to the Royal Society which contained a discussion of all the observations calculated to throw light upon the subject; Mayer also arrived at the conclusion that the Moon's motion had continually been accelerated from the earliest records. Both these astronomers found that the same mean motion of the Moon could not satisfy both modern observations and the records of the eclipses observed by the Chaldeans and Arabs. They attempted to represent these by adding to the mean longitude of the Moon a quantity proportional to the square of the number of centuries before or after 1700. "Now the lunar acceleration having been incontestably established, it was an object of great interest to ascertain the cause of this inequality. Euler came to the conclusion that it could not be produced by the force of gravitation. Lagrange at a later period demonstrated that neither the figure of the Moon, nor that of the Earth, nor the attractions of the planets, could be the cause of the phenomenon. "After many unsuccessful attempts to account for this phenomenon, Laplace at last succeeded in mastering the difficult problem, which had baffled so many distinguished mathematicians and had escaped the sagacity of Lagrange, who had almost touched it with his finger (says M. Delaunay). |
