August 2, 1894] 



NATURE 



321 



other satellites.'' He further goes on to say that the disc 

 gradually began to lengthen again and assume the ellip- 

 tical form. From these observations it was concluded that 

 the first satellite had the form of a prolate spheroid or 

 ellipsoid, or, in other words, was egg-shaped. A week 

 later each of the other satellites had been recorded to 

 some extent elliptical. Curious to relate, the three outer 

 satellites, according to the observations, appeared 

 shortened equatorially, not lengtliened, showing that they 

 did not seem to revolve round their minor axes. 



Itwasnot surprising to notice that Prof Pickering was at 

 first rather sceptical about the truth of these observations, 

 and assuming that they might be produced optically or 

 otherwise, he employed every method which would elim- 

 inate such ambiguity. These researches gave a negative 

 result. It occurred to him, also, that the effects seen 

 might be due to light and dark spots, suitably placed 

 upon the surface, but during the time of the satellites' 

 transits, and when they were about to disappear, no 

 such spots were seen, although some surface markings 

 on the first, third, and fourth satellites have been 

 discovered. 



The results to be gathered from the observations made 

 up to this time may be summed up as follows : — 



i) The first satellite is a prolate ellipsoid revolving 

 about one of its minor axes in a period of i3h. 3m. ; and 

 (2j the discs of the second, third, and fourth satellites at 

 regular intervals assume the forms of ellipses, and these 

 '■ periodic changes are presumably produced, as is thought, 

 by rotations upon their axes. 



The second contribution on the form of the satellites 

 appeared in the May number of the same journal for the 

 . following year, and Prof Pickering opens with the state- 

 j ment that " what have appeared to be most natural 

 suppositions have been found so frequently to be con- 

 tradicted by the facts, that it seemed best to take nothing 

 I for granted with regard to them." The observations here 

 deal first with the direction and period of rotation, giving 

 as a result a probable retrograde motion of rotation for 

 the first satellite. 



With regard to the change of forms, it was noticed that 

 the first satellite on January 13 appeared distinctly 

 shortened equatorially when at its minimum phase, the 

 phenomenon lasting thirty-four minutes ; while the seccnd 

 satellite is occasionally described as appearing long, like 

 the first. At the maximum phase the second, like the 

 two outer satellites, has appeared round ; and the third, 

 at som.e of its minima retains the elliptical phase for a 

 longer time than at others. Thus, for instance, it re- 

 mained short for the three days, January 13, 14, and 15, 

 consecutively. 



The next account of further observations are 

 contained in a long paper entitled " The Rotation 

 of Jupiter's Outer .Satellites," which appeared in the 

 following June number. The observations here are 

 given more in detail, but we will confine ourselves to the 

 main points, commencing with those relating to the 

 largest and most easily observed of the group, i.e. the 

 third. This satellite, according to Prof Pickering, pre- 

 sents an elliptical phase twice during its revolution in its 

 orbit at an interval of about thirty-four hours after pass- 

 ing conjunction. When on the eastern side it presents 

 an elliptical disc, and the inclination of the major axis to 

 the orbital planes is clearly marked and has been 

 measured on several occasions, the mean value being 

 - lo'5 P. angle. We have here also some important 

 observations of details visible on this disc of the satellite, 

 which, as Prof Pickering says, " can without much 

 difficulty be made out." 



A careful study shows that the marking usually appears 

 forked, and is sometimes turned to the right and some- 

 times to the left, and at other times it is seen double, 

 appearing like the letter X turned on one side. Another 

 appearance of the markings on the surface of the same 



NO. 1292, VOL. 50] 



satellite is as if an equatorial belt on one side had been 

 drawn out in both directions of the poles, its breadth 

 increasing as the limb is approached, being in fact 

 trumpet-shaped. A measurement of the position angle of 

 the axis of the belt gave + is°5. 



Special precautions were taken to find out whether the 

 belt were a genuine phenomenon, or an illusion due 

 either to the instrument or the observer, but these 

 resulted in it being declared real. The observations 

 for the determination of the rotation of this satellite 

 implied a period of rotation coinciding with that of the 

 revolution of the satellite in its orbit. 



A recapitulation of the facts relating to the third 

 satellite observed up to this time may be made 

 here : — 



Two observers see the disc flattened at regular 

 intervals, and agree upon direction of flattening. Both 

 see a belt in same position, direction, and character of 

 detail, and both remark that the observations are not 

 very difficult, but quite evident when attention is called 

 to them. 



The details in the other satellites seem to be more 

 difficult to see than those on the third. The direction of 

 the bands (one or two) in the first satellite lies in an 

 approximately north and south line, while on only 

 one occasion there was detail detected on satellite II., 

 and this consisted simply of a small patch or spot. 



The observed facts with regard only to the forms of 

 these discs are : — 



The shape of the first is elongated. The regularly 

 recurring changes of shape of the discs of the outer 

 satellites (caused apparently by rotation). The change 

 of position angle of the major axis of the third, and 

 probably of the fourth in different parts of their orbits. 



The peculiar behaviour of satellite III., which in 

 October and November was recorded as shortened 

 in the polar direction, but which was afterwards recorded 

 upon two nights as perfectly round, when it should have 

 I exhibited its maximum ellipticity. The frequently 

 recorded lengthening (equatorially) of satellite II., which 

 is not corroborated by recent observation. .Apparent 

 irregularities in period and ellipticity of the second and, 

 perhaps, the fourth, and the occasional irregular non- 

 elliptical shape of the disc of the third. 



Such, then, is the sum total of the observations which 

 up to that time had been made by Prof Pickering, and 

 we will now turn our attention to the re-examination of 

 the satellites by Prof Barnard with the help of the great 

 Lick refractor. {Astronomy and Astro-Physics for April, 



1894). 



He commences by telling us that the satellites I., 

 III., and IV. often undergo singular transformations 

 of apparent form during certain stages of their transits 

 across the face of Jupiter, but he had never " seen any of 

 these moons other than round when off the disc of the 

 planet.'' 



In the latter end of the year 1S93 to the beginning 

 of 1S94, with a power of 1000 diameters, and some- 

 times higher, he made numerous observations, with the 

 object of detecting, if present, deviations from the disc 

 forms. 



The results showed that no such deformations were 

 observed, and Prof Barnard is inclined to think that the 

 surface markings of the satellites themselves, when near 

 the edges of the discs, might readily cause apparent dis- 

 tortions in these satellites, as they certainly do so when 

 the satellites are in transit, especially when very high 

 magnifying powers on a small telescope are employed. 

 It iscurious, he adds, that such deformations should escape 

 detection with our great telescope, even with the most 

 casual observations, considering how conspicuous are 

 the distortions recorded at Arequipa. It is of interest 

 to note one or two remarks accompanying some of the 

 observations, thus : — 



