Dec. 9, 1880] 



NA TURE 



137 



suggested. The experience of the transit of 1S74 poiats to tlie 

 desirability of sacrificing something in the magnitude of the 

 parallax-factor for the sake of securing a higher elevation of 

 the sun ; thus, for retarded ingress, Sir George Airy had at 

 first proposed to refer principally to the coasts of the Canadian 

 Dominion and the United States of North America, where the 

 sun's elevation is from 15° to iS' ; he now proposes to substitute 

 for this the whole chain of West India Islands, from the eastern 

 extremity of Cuba to Barbadoes, or stations on the neighbouring 

 continent of Central America. Bermuda is also included as a 

 favourable point for observation. Most, if not all, of the 

 longitudes required have been determined with great precision 

 by the Hydrogi-aphic Department of the United States. For 

 ingress accelerated. Sir George Airy relies entirely upon stations 

 in the Cape Colony. For the accelerated egress, all the stations 

 suggested for mgress retarded will be available. For egress 

 retarded, although the fixed Observatories at Melbourne and 

 Sydney will contribute to the observation of the phenomenon, 

 they will have the sun at a somewhat low elevation (10 — 14°) ; 

 it is thereby proposed to rely mainly upon New Zealand, with 

 which we are in telegraphic communication via Sydney. Con- 

 siderable correspondence has taken place on the subject of 

 Australian longitude, and it is expected that the necessary steps 

 to effect the connection of one of the Observatories, probably 

 Adelaide, with Madras, will be taken early in the ensuing 

 year. 



Sir G. B. Airy has completed the laborious calculations in 

 his Numerical Lunar Theory, from which the corrections to the 

 coefficients of Delamiay's Lunar Theory are to be deduced ; 

 and in connection with this work he has made an investigation 

 of the value of the Moon's Secular Acceleration, for which he 

 finally obtained the value 5"'477, thus confirming the results 

 obtained by Prof. Adams, and subsequently by Mr. Delaunay. 

 On this important question. Prof. Adams has also published an 

 investigation [Mont/ily iVaticcs, vol. xl. Nos. 6, 7, S and 9). 



A new determination of the Physical Libration of tlie Moon 

 from a large number of lunar photographs taken with the 

 De La Rue reflector at the Oxford L^niversity Observatory has 

 been recently made by Prof. Pritchard, the result being to 

 indicate the existence of a small rotational inequality. 



Messrs. J. Campbell and Nelson have made use of the 

 Greenwich Obsenations, 1S62 to 1876, to determine tlie Lunar 

 Parallactic Inequality, from which they deduce for the value of 

 the Solar Parallax, 8"'77S, or S"'84S, according as the existence 

 of a forty-five year inequality, apparently indicated by the ob- 

 servations, is admitted or not (Monthly Notices, vol xl. Nos. 7 

 and S). The Sun's Parallax has also been determined by Mr. 

 Downing, from N.P.D. observations of Mars at Leyden and 

 Melbourne, in 1S77. The value thus found is S"'96 {Astrono- 

 misc/ic Nachfichtai, No. 22SS). 



In continuation of his researches on tidal retardation from the 

 action of a satellite on a viscous planet, Mr. G. H. Darwin has 

 investigated the secular changes in the orbit of a satellite, deducing 

 the early history of the earth and moon from the time when they 

 were initially in contact, each revolving in the same period of 

 from two to four hours. This leads to the suggestion that the 

 moon •\\as produced by the rupture of the primceval planet. In 

 another memoir, Mr. G. H. Darwin gives analytical expressions 

 for the history of a planet and a single satellite. {Phil. Trans., 

 1879, Proc. Roy. Soc, Nos. 203 and 202.) 



An important work in connexion with the United States 

 Northern Boundary Commission has been ]3ublished by Mr. 

 Lewis Boss, on the Declination of Fixed Stars. The systematic 

 corrections to some seventy catalogiies have been discussed, and, 

 from the mean of the whole, standard declinations of 500 stars 

 have been deduced. 



Dr. Gould's " Uranometria Argentina" and M. Houzeau's 

 " Urauometrie Generale," are of especial value as giving im- 

 jportant information on the brightness and distribution of the 

 stars in the southern hemisphere. 



Interesting results as to the diameters of satellites have been 

 obtained by Prof Pickering from photometric observations, 

 On the assumption that their albedos do not dift'er greatly from 

 those of their respective primaries. (Annals, Harvard College 

 Observ.atory, vol. xi. ) He has further investigated, on some- 

 what similar principles, the dimensions of the fixed stars, with 

 especial reference to binaries and variables of the Algol type. 

 (Proc. Anier. Acad., vol. xvi.) Prof. Pickering has also 

 commenced a photometric survey of the heavens, in which the 

 brightness of every star visible to the naked eye is to be deter- 



mined. He has further undertaken a search for planetary 

 nebuljE by a new method, in which, by the use of a direct-vision 

 prism in front of the eyepiece, the nebula is at once detected by 

 its monochromatic spectrum, focussing a point of light instead of 

 a colom-ed line as in the case of a star. About a hundred thousand 

 stars have been examined, and four new planetary nebula; 

 have been detected. (American Journal of Science/ October, 

 1880.) 



From the grouping of the aphelia of certain periodic comets 

 Prof. G. Forbes has inferred the existence of two ultra- 

 Neptunian planets, and has indicated their approximate jiosi- 

 tions. (Trans. Roy. Soc, Edinburgh.) Mr. D. P. Todd has 

 deduced from the perturbation of Uranus a position for an 

 ultra-Neptunian planet closely agreeing with that found by Prof. 

 G. Forbes. So far, the search for the hypothetical planet with 

 the 25-inch Washington refractor has been unsuccessful. 

 (American Journal of Science, September, 1S80.) 



Prof. BreHichin's researches on the tails of comets have led 

 him to the classification of these appendages according to the 

 value of the solar repulsive force which v.ould have generated 

 them. Having di cussed the forms of the tails of thirty-three 

 comets, he finds thii they belong to three types, corresponding 

 respectively to repulsive forces II, i"4 and o"3 (the sun's gravi- 

 tation being taken as 1) ^ and adopting Zoilner's hypothesis of a 

 repulsive force, due to electricity and inversely jDroportional to 

 the specific gravity, he infers that the tails of the three types 

 are composed respectively of hydrogen, carbon, and iron. In 

 the case of the second and third t)pes other elements of nearly 

 the same atomic weight may replace or be mixed with the carbon 

 and iron, and in such a comet as Donati's a number of sub- 

 stances may be mixed in the tail, which will consequently spread 

 out in the plane of the orbit. The first type composed of 

 hydrogen will always remain separated from the others. 

 (Annales de I'Observatoire de Moscou, vols, iii-vi.) 



The appearance, at the beginning of this year, of a great 

 comet in the southern hemisphere, recalling by the length of its 

 tail and thesmallness of its head the remarkable comet of 1843, 

 has excited great interest, more especially as it was found that 

 the orbits of the two comets were sensibly the same. The 

 observations of the comet of 1S43, however, do not appear to 

 be compatible with so short a period as thirty-seven years, and 

 Prof. Oppolzer has shown that the action of a resisting medium 

 would not meet the case. (As/ronomische Nachrichtcn, Nos. 

 2314, 2315.) Uuder these circumstances Prof. D. Kirkwood 

 has suggested that the two bodies may be fragments of one 

 original comet, viz., that of 370 B.C., which is said to have 

 separated into two parts like Biela's comet (Observatory, No. 

 43.) Five other comets (including Faye's periodical comet) 

 have been discovered this year, but two of them were lost 

 through cloudy weather before a second observation could be 

 obtained. 



In astronomical physics Mr. Huggins has obtained photo- 

 graphs of stellar spectra, which establish the existence of a 

 remarkable group of nine bands in the ultra-violet, probably due 

 to hydrogen, and further lead him to an arrangement of the 

 stars in a continuous series according to the breadth and mar- 

 ginal difference of the typical lines, particularly of the K line. 

 Mr. Lockyer continues his researches on dissociation, as indi- 

 cated in solar outbursts, and in connection with this work is 

 engaged on a systematic observation of the spectra of sun-spots. 

 At the request of the Committee on .Solar Physics, correspond- 

 ing observations are being made at Greenwich. ' 



From the series of Greenwich photogi-aphs of the sun, 1S74 

 — 1879, the mean heliographic latitude of spots and mean 

 distance from the sun's equator, have been deduced for each 

 rotation and for each year ("Greenwich Spectroscopic and 

 Photographic Results," 1879). 



A fine 36-inch silver-on-glass reflector has been recently con- 

 structed by Mr. Common, and with this instrument he has 

 obtained photographs of Jupiter, showing the red spot, and of 

 the satellites (Obserjatory , No. 34). 



At the outset of an undertaking one figures to oneself in 

 imagination what may be done ; towards the close of it one sees 

 in actual fact what has been done. In commencing this address 

 I had hoped to say something of the progress of mathematics ; 

 before bringing it to a conclusion I find my space filled and my 

 time exhausted. How far the good intentions of this year may 

 be realised in the next, cannot yet be seen ; but the difficulties of 

 a task do not always diminish the fascination of making an 

 attempt. 



