3 o8 



NA TURE 



\_July 26, 1883 



OUR ASTRONOMICAL COLUMN 



The Ellipticity of URANUS. — It may be remembered that 

 Sir William Herschel, who was at first under the impression that 

 the disk of Uranus presented a perfectly circular outline, was 

 afterwards convinced that there was an appreciable elongation 

 in the direction of the major-axis of the orbits of the satellites, 

 though he has not recorded any measures to test this conclusion. 

 On October 13, 1782, about eighteen months after the discovery 

 of the planet, he writes: "I perceived no flattening of the 

 polar regions." On March 5, 1792, he used "a newly polished 

 mirror of an excellent figure : it showed the planet very well 

 defined and without any suspicion of a ring." With powers 

 240-2400, all which his speculum bore with great distinctness, 

 he formed a different opinion, and remarked, "I am pretty well 

 convinced that the disk is flattened." On February 26, 1 794, he 

 has an observation thus recorded, "20-feet reflector, power 480, 

 The planet seems to be a little lengthened out in the direction of 

 the longer axis of the satellites' orbits." Further, in a paper 

 communicated to the Royal Society in December, 1797, where- 

 in he announces his supposed discovery of four additional 

 satellites of Uranus, he says : " The flattening of the poles of the 

 planet seems to be sufficiently ascertained by many observations. 

 The 7-foot, 10-foot, and the 20-foot instruments equa'ly confirm 

 it, and the direction pointed rut February 26, 1794, seems to be 

 conformable to the analogies that may be drawn from the situa- 

 tion of the equator of Saturn and of Jupiter." This ellipticity 

 being admitted, he inferred that Uranus had a rapid axial 

 rotation. 



In September, 1842, Madler, remarking that notwithstanding 

 the statement mnde by Sir W. Herschel no measures of the 

 planet existed which would confirm it or otherwise, instituted a 

 series with the filar-micrometer of the Dorpat refractor. The 

 measures were made on five nights, and the diameter of the 

 planet was determined at every 15° of the circumference, the 

 mean of each set being made to fall nearly at the time of meri- 

 dian passage. The nights (September 16, 17, 19, 20, and 21) 

 were of exceptional clearness, and permitted of a power of 

 1000 being used. Madler found the greater diameter of Uranus 

 4"'249 at the planet's mean distance, and the compression 



-- ; the angle of the greater axis was 160° 40' counted from 

 10-85 



north towards east. At this time Uranus was less than 11° 

 from the descending node of the orbits of the satellites, as deter- 

 mined by Prof. Newcomb. 



Between August 24 and October 20, 1843, Madler repeated 

 his measures on seven nights : his results from this year's series 

 v\ ere — 



Greater axis of projected ellipse 



Lesser axis ,, ,, 



Compression 



Angle of greater axis with declina- 

 tion circle 



4-3274 



3-8910 



1 



... i5°26'-i 



This ellipse is for September 28, 1843, when the distance of 

 Uranus was 19-079. The greater axis for the mean distance of 

 Uranus would be 4"'304. 



An ellipticity comparable with that of the planet Saturn might 

 have been expected to strike the generality of observers provided 

 with the large instruments which have been available since the 

 e, och of Madler's measures ; yet neither with the Pulkowa re- 

 fractor, with the late Mr. Lassell's 4-foot reflector, employed by 

 him and Mr. Marth in measures of Uranus at Malta in 1864-5, 

 nor with the Washington 26-inch refractor, or many other instru- 

 ments of adequate power, do we find that there has been any 

 confirmation of the great inequality of diameters found by 

 Madler, up to 1877. 



It now app.ars from a communication made by Prof. Safarik 

 of Prague to the Astronomische Nachrichtcn in April last, that 

 on March 12, 1S77, he found Uranus "certainly elliptical, the 

 greater axis in the parallel," and this impression he received on 

 various occasion, up to the date of his letter. On April 2 in 

 the present year he records of the appearance of the planet : 

 " Stets stark langlich ; in den beslen Mouienten schiitze ich die 

 Elliptitat starker als jene Saturns" ; the greater axis was at 

 190°. The instruments used were of very moderate capacity, 

 being an achromatic of II cm. and a silver-on-glass speculum 

 of 16 cm. 



In consequence of a representation from Prof. Safarik, who 

 laid stress upon the actual proximity of the planet to the ascend- 



ing node of the orbits of the satellites, Prof. Schiaparelli has 

 made, this year, an extensive series of measures of the diameter 

 of Uranus, the results of which have appeared in No. 2526 of 

 the above-named periodical. The measures are di-cussed on 

 two methods giving for the ellipticity of the planet in the one 



case , and in the other (perhaps the more preferable 



10-98 ±0-93 r ' 



value), _ . In addition to actual measures, Prof. 



io-94±o-67 

 Schiaparelli drew the outline of the planet, as it appeared to the 

 eye, on thirteen nights, the drawings giving by measurement an 



ellipticity of . An assistant in the same way found 



u 07 ' 



The Milan measures with the filar-micrometer were 



109 



made between April 12 and June 7. For the equatorial dia- 

 meter at the mean distance Prof. Schiaparelli found 3"'9ii. 



PHYSICAL NOTES 



In the current number of Wiedemann's Anna/en, Prof. C. 

 Christiansen of Copenhagen resumes his researches on the 

 indices of refraction of coloured liquids. The methods adopted 

 consisted in the examination of the liquid in hollow prisms of 

 very small refracting angle ; a few drops of the liquid being 

 placed bttween two small pieces of glass touching each other at 

 one side, but separated about half a degree. Another method 

 consisted in inclosing the liquid between a piece oi very thin 

 glass and a biprism made of a glass the index of refraction of 

 which was known, the index of the liquid being calculated by 

 taking the refraction as the oifierence of the two separate refrac- 

 tions of the glass and the liquid. Prof. Christiansen gives 

 tallies of results for water, alcohol, turpentine, and nitrobenzol, 

 and also for solutions of permanganate of potash of various 

 degrees of concentration. For the latter substance the results 

 agree with the determinations of Kundt, but are probably mo.e 

 exact. 



Prof. G. M. Minchin has greatly improved the form of the 

 ab olute sine electrometer invented by him some months ago. 

 The first of the new instruments constructed by Mr. Groves of 

 Bolsover Street is now complete, and is to be sent out to Prof. 

 Anthony of the enterprising and wealthy Cornell University. 

 We hope shortly to illustrate and describe this beautilul 

 instrument. 



Prof. Ewing of Tokio prints in the Proceedings of the Seismo- 

 logical Society of Japan three valuable seismological notes. The 

 first of these describes a duplex pendulum seismometer the prin- 

 ciple of which is the following : — A common pendulum having 

 its centre of gravity below the centre of suspension is stable ; an 

 inverted pendulum with pivoted supporting rod is unstable. By- 

 placing an inverted pendulum below a common one, and con- 

 necting the bobs so that any horizontal displacement must 1 e 

 common to both, the equilibrium of the jointed system may be 

 made neutral or as nearly stable as is desired. A very sensitive 

 seismograph is thus obtained. The instrument has not yet been 

 put to the test of an actual earthquake. 



PROF. Quinckk has contributed to the Proceedings of the 

 Royal Prussian Academy of Sciences an important memoir on the 

 changes produced by hydrostatic pressure in the volume and 

 refractive index of transparent liquids. The ratio of these 

 changes exhibits, it appears, a definite relation. The compressi- 

 bility in volume was measured by subjecting the liquids to pres- 

 sure in glass vessels furnished with capillary tubes. The indices 

 of refraction were measured by observing the number of inter- 

 ference bands in homogeneous light in an interferential rcfracto- 

 meter. One of the most important re-ults of this research is the 

 light it throws on the disputed formula called the constant oj 

 refraction. According to Dale and Gladstone the name of 

 constant of refraction, or specific refractive power, should be 



assigned to the quantity M ~~ , where p is the index of refrac- 

 tion and s the specific gravity of the substance. According, 

 however, to Laplace the quantity ^— - — is the true constant of 



refraction ; whilst, according to Professors II. A. and L. Lorenz, 

 that name should be given to the more complicated function 



** ~ ' . Now since with liquids that are subjected to pres- 



ry + 2) s 



