Sept. 9, 1875J 



NATURE 



397 



port and a health-resort in one borough, and which, therefore, 

 ini;_'ht be taken into account in any deductions from statistics of 

 health or mortality of their united populations. 



British Association, Bristol W. J. Black 



A Lunar Rainbow ? 



Theke can be little doubt that your Australian correspondent, 

 Mr. Lefroy (vol. xii. p. 329), has seen one of the phases of an 

 Aurora Auftralis. Similar appearances have been observed by 

 me in Scotland, passing south of the zenith (and nearly through 

 the anti-dip, as at Fremantle). Their sudden occurrence and 

 temporary persistence are perplexinp to those who have not seen 

 this particular display before. The first seen by myself (in 1844, 

 I think) was a single beam which remained in the same position 

 during some hours ; it was described by me next day in a local 

 paper, while a well-known observer in a communication to an 

 Edinburgh journal had taken it for a comet. 



It is pleasant to see accounts of such phenomena sent to 

 N.^.TURE from all parts of the world, even when the true cause 

 has not always been apparent. It is not improbable that the 

 magnets at Melbourne will have shown some slight disturbance 

 about 8h. 30m. P.M. of May 16. 



John Allen Broun 



I DO not see any reason to doubt that the phenomenon seen 

 by "J. W.N. L." in Australia, and described by him in vol. xii. 

 p. 329, was an aurora, I never saw one with so many arches as 

 he mentions (eighteen or twenty), but there can be no reason for 

 supposing so large a number to be impossible. In almost every 

 other respect his description agrees exactly with auroras such as 

 may occasionally be seen. T, W, Backhouse 



West Hendon House, Sunderland, Sept. 4 



The House-Fly 

 I WAS somewhat interested in Mr. Cole's remarks on the house- 

 fly in Nature (vol. xii. p. 187), and recently had an example 

 of another of its enemies. On touching a rather small decrepit 

 house-fly which was making its way across a sheet of paper, 

 three minute, active animals, apparently beetles, tumbled out of 

 it ; they were light brown in' colour, and very much the shape of 

 aphides, and about the size of the hole a medium sized pin would 

 make when pushed through paper. F. P. 



OUR ASTRONOMICAL COLUMN 



M. Levekrier's Theory of Saturn.— Early in the 

 year 1874, M. Leverrier presented to the Paris Academy 

 of Sciences the conclusions he had drawn from the com- 

 parison of his analytical theory of the planet Jupiter with 

 the meridian observations made at Greenwich and Paris 

 during the long period of 120 years, which he found 

 to be represented thereby with all desirable precision ; 

 thus proving that the motion of Jupiter is not subject to 

 any sensible action beyond the effects of the known 

 planets. 



The comparison of the theory of Saturn with a similar 

 extended course of normal positions, each one based upon 

 a great number of observations, has not run quite so 

 smoothly, but, on the contrary, has presented some slight 

 difficulties, upon which M. Leverrier makes known his 

 opinion, in a communication to the Paris Academy on 

 the 23rd of last month. During the thirty-two years of 

 modern observations, 1837-69, the differences between 

 theory and calculation, except in two instances, remain 

 below o'2S, in the times of passage observed on the 

 meridian ; for the older observations of Maskclyne and 

 Bradley, somewhat larger discordances are shown. The 

 residuals are, however, upon the whole, very small, and a 

 question arises, whether such quantities can be legitimately 

 neglected, or, if not, whether their cause is to besought 

 in incompleteness of the analysis or in errors of the 

 observations themselves. M. Leverrier has not been 

 content to rest upon the first supposition, but states that 

 he has used every effort to elucidate the source of the 



remaining differences. To satisfy himself and astro- 

 nomers generally that there is no defect or inaccuracy of 

 theory, M, Leverrier has taken extraordinary pains to 

 guard against error or omission. When he found in his 

 earlier researches a discordance between theory and ob- 

 servation in the case of Mercury, he was able to explain 

 the whole by admitting an increase in the motion of the 

 perihelion, which might be attributed to the existence of 

 cosmical matter or the action of small bodies nearer to 

 the sun than the planet ; and again, when the comparison 

 of theory with the observations of Mars showed differ- 

 ences, they were explainable by a similar assumption of in- 

 creased motion of the perihehon, necessitating an increase 

 in the mass of the earth, and consequently of the solar 

 parallax. In the case of Saturn, the smallness of the 

 residuals has rendered it a much more difficult matter to 

 pronounce with confidence upon their cause. Having' 

 reviewed the whole of his analytical theory, M. Leverrier) 

 with the view to further verify it, considering this 

 theory as a first, though exact approximation, pro- 

 ceeded by methods of interpolation to reconstruct it, 

 taking account at once of the terms of all orders. 

 Every possible verification having been thus accumu- 

 lated, he concluded that no error was to be appre- 

 hended in this direction. The comparison with the 

 normal positions having been certified with equal care, 

 he ascertained the effect of small changes in the masses 

 of Jupiter and Uranus, the errors being exhibited in func- 

 tions of the corrections to these masses, and the results 

 prove that no alteration in the adopted value of either 

 mass will destroy the residuals as a whole ; if they are 

 somewhat diminished thereby in one part of the series, it 

 is only at the expense of increasing them in other parts. 

 Indeed, M, Leverrier establishes one point, and a very 

 remarkable one it will no doubt be considered, viz., that 

 the 120 years of meridian observations of Saturn are 

 insufficient to afford a reliable value of the mass of 

 Jupiter ; or, in other words, that the mass of Jupiter which 

 has so great an importance in the elements of the solar 

 system, is not yet determinable from the comparison of 

 the theory of Saturn with observations. This was not the 

 case as regards the mass of Saturn, which M. Leverrier 

 found from his researches upon the motion of Jupiter to 



be 



a somewhat smaller value than that resultin'. 



3529-56 

 from Bessel's measures of the Huygenian satellite. 



Under the above circumstances, the probability that 

 errors of observation are the cause of the remaining 

 differences from theory is much increased, and M. 

 Leverrier appears inclined to attribute these errors to the 

 interference of the rings under their various phases, an 

 explanation which practical men will assuredly regard 

 with favour. Considering that at certain times the rings 

 disappear entirely, when the planet's centre may be well 

 observed, while at others intervening in an elliptical form, 

 projecting shadows and occasionally rendering impossible 

 the observation of one of the limbs, there is nothing un- 

 likely, as M. Leverrier remarks, in an uncertainty of some 

 tenths of a second in R.A., which would sufficiently ex- 

 plain all. At any rate, whatever influence the interference 

 of the rings may have upon the observations, it is doubtless 

 of a variable character, as well on account of the physical 

 fact itself, as from the effect it may exercise on personal 

 equations. 



Mr, De la Rue's Tables for Reduction of Solar 

 Observations, — " Auxiliary Tables for determining the 

 angles of position of the Sun's Axis and the Latitude and 

 Longitude of the Earth referred to the Sun's equator," 

 which have been employed in the reduction of the ten- 

 year series of solar photograms taken at the Kew Obser- 

 vatory, have just been printed by Mr. De la Rue, pro- 

 fessedly for private circulation, though, as they have been 

 imposed in the size and type of the " Philosophical Trans- 



