236 



NATURE 



\yuly 4, 1889 



Manheim, estimated it to be as high as 10" (Grant's " History of 

 Astronomy," p. 275). 



The difference between these measures of the diameter of 

 'Uranus would seem to be a consequence of the fact that it is 

 -always extremely difficult to get the planet clearly defined in the 

 'field of the telescope. Herschel himself noted ( Phil. Trans. , 1 798, 

 p. 69), "The Georgian planet is not so well defined as, from 

 the extraordinary distinctness of my present 7-feet telescope, it 

 ought to be. There is a suspicion of some apparatus about the 

 planet." 



Herschel several times had the impression that Uranus was 

 -surrounded with a ring. One of his observations is contained in 

 the following (Philosophical Transactions, 1798, p. 68): — "My 

 telescope is extremely distinct, and when I adjust it upon a very 

 minute double star, which is not far from the planet, I see a very 

 faint ray, like a ring crossing the planet, over the centre. This 

 appearance is of an equal length on both sides, so that I strongly 

 suspect it to be a ring. ... I have turned the speculum one 

 quadrant round, but the appearance of the very faint ray con- 

 ''tinues where it was before, so that the defect is not in the 

 speculum nor is it in the eye-piece." Later observations, how- 

 ever, led Herschel to conclude "that Uranus has no ring in the 

 least resembling that, or rather those, of Saturn." 



Following upon the observations as to the existence of a ring 

 'round Uranus, are found others relating to its polar compression. 

 The flattening at the poles was first observed by Herschel in 

 February 1794 (Phil. Trans., 1798, p. 69), and announced in the 

 following words : " The planet seems to be a little lengthened 

 out, in the direction of the longer axis of the satellite's oi"bit ;" and 

 again in April of the same year : ' ' The disk of the planet seems to 

 be a little elliptical." Madler measured the ellipticity in 1843, 

 and found it i/9'92 (Grant's "History of Astronomy," p. 278). 

 Schiaparelli, in 1883, using two different methods, obtained the 

 results 1/10*98 and i/io'94 {Astr. Nach., No. 2526). A few 

 measures made by Young in the same year gave an ellipticity 

 1/14 ("General Astronomy," Young, p. 367). The fact that 

 the ellipticity was in the same plane as the major axis of the 

 satellite's orbit led Herschel to conclude from analogy with 

 Jupiter and Saturn, " That the Georgian planet also has a rota- 

 tion upon its axis of a considerable degree of velocity " (Phil. 

 Trans., 1798, p. 71). Other observers of the bulging out of the 

 'Uranian equator — Schiaparelli, Young, Safarik — agree with 

 Herschel in saying that the plane is coincident v/ith that of the 

 satellite's motion, but the following observations of markings on 

 the surface of Uranus lead to an entirely different conclusion. 



Buff ham noticed some bright markings on Uranus in 1870-72 

 {Monthly Notices, vol. xxxiii. p. 164), and, from observations of 

 ■their motion, deduced the time of rotation as twelve hours, but 

 the plane of rotation was not coincident with that of the satel- 

 lite's orbit. This was borne out by observations of dusky bands 

 by Young, in 1883 (" Princetown Observations," 1883) ; of ap- 

 parent equatorial belts by the brothers Henry, in 1 884 ( Comptes 

 rendus, t. xcviii. p. 1419) ; and observations in 1884, at Nice, of 

 a bright spot by Lockyer, Perrotin, and ThoUon {Comptes rendus, 

 t. xcviii. pp. 717, 967). The plane of rotation, according to 

 these observers, is from 15° to 40° from the trend of the satel- 

 lite's. Thus the difference between the two sets of observations 

 amounts to nearly half a right angle. Does the error lie in the 

 observation of the belts, or in the measurements of the planet's 

 ellipticity and the satellite's orbit ? This is an enigma which 

 yet remains to be solved, and another character of Uranus re- 

 quiring investigation. 



Her>chel made the first determination of the mass of Uranus 

 in 1788 (Philosophical Transactions, 1788, p. 369), and found 

 rit to be 17740612 as compared with the earth, or about 

 i/i8,ocb that of the sun. Bouvard found a value 1/17,918 ; 

 Lamont, in 1837, 1/24,605. Lassell's observations of the motion 

 •of the satellites gave a value 1/20,897, whilst Struve's obser- 

 vations gave a value 1/26,860. The mass, 1/22,600, found by 

 Newcomb (" Washington Observations," 1873), is probably the 

 most correct, and he estimates that the probable error in the 

 denominator is not more than 100. This mass, revolving round 

 ;the sun at a mean distance of about 1800 millions of miles, 

 must exert considerable influence upon bodies near it, influence 

 which may often predominate over that of the sun. 



But it is the question of Uranian satellites that is so enig- 

 matical. Herschel discovered two on January 11, 1787 (Phil. 

 Trans., 1787, p. 125 et scq.), and in 1798 announced the dis- 

 covery of four more. Regarding the real existence of these four, 

 Herschel remarks (Phil. Trans., 1798, p. 66) : " It remains now 



only to be mentioned that, in such delicate observations as these 

 of the additional satellites, there may possibly arise some doubts 

 with those who are very scrupulous ; but as I have been much 

 in the habit of seeing very small and dim objects, I have not been 

 detained from publishing these observations sooner, on account of 

 the least uncertainty about the existence of these satellites, but 

 merely because I was in hopes of being able soon to give 

 a better account of them, with regard to their periodical 

 revolutions." 



Sir John Herschel observed the two brightest satellites be- 

 tween 1828 and 1832 (Mem. Ast. Soc, vol. viii. p. i) ; but 

 "of other satellites," he says, than these, "I have no evi- 

 dence," although the telescope he was using was precisely similar 

 to that used by his father. A systematic search was made by 

 Lassell for the lost satellites, and a definite announcement of 

 the discovery of two satellites between Uranus and the two 

 brightest was made in 185 1 {Monthly Notices R.A.S., xi. 

 248). He declares, however, that it would have been impos- 

 sible for Sir William Herschel to have seen these two faint 

 bodies ; and although Prof. Holden has attempted to identify 

 the two with two of Herschel's quaitett, the balance of evidence 

 is certainly to the contrary, and we are bound to conclude that 

 no one has ever seen the four but Herschel himself. 



Herschel announced, in 1798 (Phil. Trans., 1798, p. 48), the 

 retrograde movements of the Uranian satellites in the terse 

 paragraph, "I take this opportunity of announcing that the 

 movement of the Georgian satellites is retrograde." The fact 

 that their orbits were inclined about 80° to the ecliptic plane 

 was discovered in 1788. 



Herschel also particularly noticed that the light of the two 

 brightest satellites was subject to considerable fluctuations, and 

 in 1815 (Phil. Trans., 1815, p. 356) he suggests for a cause that 

 given by Newton in the " Principia" to account for the periodical 

 variability of certain stars. His conclusion was : — " The variable 

 brightness of the satellites may be owing to a rotation upon their 

 axes, whereby they alternately present different parts of their 

 surface to our view. These variations may also arise from their 

 having atmospheres that occasionally hide or expose the dark 

 surface of their bodies, as is the case with the sun, Jupiter, and 

 Saturn." 



The two inner satellites, Ariel and Umbriel, discovered by 

 Lassell, seem also to fluctuate in brightness, and Newcomb 

 observed in 1875 (" Washington Observations," 1873, p. 43) : — 

 " I strongly suspect that Ariel, at least, belongs to that class of 

 satellites of which the brilliancy is variable and dependent on its 

 position in its orbit. The evidence of variability of some kind 

 seems indisputable, as I have repeatedly failed to see it when 

 the circumstances, distance from the planet included, were in 

 every respect favourable, and when Umbriel, though less favour- 

 ably situated, was visible. On the other hand, there were two 

 occasions, January 28, 1874, and March 25, 1875, when it was 

 surprisingly conspicuous. Unfortunately no systematic record 

 was made of the times when, being near greatest elongation, it was 

 looked for and not seen ; but on at least one such occasion its 

 position angle was 180°. An inspection of the observations shows 

 that out of the eight observations only two were made near the 

 southern elongation ; while in the two cases where its brightness 

 was most remarkable, the position angles were respectively 348° 

 and 351°." 



Tlie time of revolution of Ariel is 2 "520378 days at a mean 

 distance of 120,000 miles ; its diameter is about 500 miles. 



Herschel also observed that these satellites became invisible 

 some distance from the planet's disk (Phil. Trans., 1798, p. 75) ; 

 thus, on February 22, 1791, the first satellite was lost when 22" 

 from the planet. This distance was not, however, constant, for 

 on May 2, 1791, the same satellite disappeared at an apparent 

 distance 19" '8. A table is given showing at what distance from 

 the planet the first and second satellites respectively became 

 invisible during a period of seven years. A fact exhibited by this 

 table is that the distance at which the satellites disappeared 

 regularly diminished from 1791 to 1797, until in the latter year 

 the first satellite was traced to 4" "8 from the planet's disk. The 

 reason assigned to account for this phenomenon by Herschel was 

 that the light of the satellites was "put out" by the stronger 

 light of their primary, and regarding this he remarks (Phil. 

 Trans., 1798, p. 78): — " We may avail ourselves of the observa- 

 tions that relate to the distances at which the satellites vanish, 

 to determine their relative brightness. The second satellite 

 generally appears brighter than the first ; but as the former is 

 usually lost farther from the planet than the latter we may admit 



