August, 1903.] 



KNOWLEDGE, 



179 



ami gaiu a clear iusight as to the topo-^'raphv of the disc ; 

 but what we discern amid Ihe envelopes may afford an 

 intimation of what is progressin<:; below. There seems 

 little doubt that frequent eruptions of material occur from 

 tEe planet, and that the belts are often reinforced or in- 

 tensified by this ejecta. A much heated condition of Jupiter 

 seems a necessary inference, and there is observational 

 jjroof that his giant sphere is. in a state of considerable 

 activity. By volcanic agency large volumes of material are 

 probably discharged outwards, and these upon entering 

 cooler regions may be partly solidified, and enabled to 

 float on the heavier lower vapours. The red spot has 

 endured so long, and retained such a permanency of shape, 

 that it must be regarded as a very compact, stable object, 

 possibly develoj)ed from some special outburst on the 

 planet's surface at a rather remote period in the past. 



More than once, remarkable outbreaks of dark spots 

 have l)een observed, and these spreading out in a longi- 

 tudinal direction have ultimately been drifted into new 

 belts by the action of the very swift rotational velocity. 

 But as all or neai-ly all the markings are influenced by 

 proper motions, it is scarcely possible to ascertain the exact 

 rotation period. There was an outburst of dark material 

 from the planet in I860 which seems to give us a good 

 clue. It was spun into a new belt on the planet in a 

 period of 64 days, but one end of the disturbance, probably 

 representing the seat of energy, maintained a constant 

 rate of 9h. 56m. Is., while the other end showed an 

 increasing velocity. If, therefore, we are justified in 

 assuming that the dark material represented an uprush 

 from the planet's surface, the value alluded to must closely 

 approximate to, if it does not coincide with, the rotation 

 period of the Jovian sphere. 



In recent years spots may be said to have chiefly 

 abounded in four latitudes, and to have given evidence of 

 marked differences in rate : — 



Latitiule. 



Mean Rotation Period. 



Notes 



+ 25° to 30" 9 55 55-3 16 determinations. 



+ 10° to 15° 9 55 31-2 35 determinations. 



Equatorial Region ... 9 50 25-6 5 years, 1898-1902. 



-25° to 30° 9 55 18-7 29 determinations. 



Red spot and hoUow in 9 55 36-56 70 years' observations, 1831- 

 belt. Section of -20° lilol. 61,813 rotation'. 



And thess particular regions of the planet are at the 

 present moment very rich in spots of various character. 

 The sedulous study of these currents, and of the objects 

 carried along in tliem, must be carefully pursued from 

 year to year, for they may ultimately teach us something as 

 to periodical changes of velocity possibly affecting them, 

 as well as enable us to recognise recurrent markings. 



In the spring of 1901 several dark projections were 

 observe 1 from the S. side of the S. equatorial belt. These 

 intcMsifiod, and a considerable amount of dark matter was 

 thereaftt-r distributed over the S. tropical zone. This 

 enveloped tlie red spot in July, 1902, and passed that 

 object l)y flowing round its southern borders. The large 

 tropical spot is still visible, though much fainter than 

 loniierly. On June 30 it extended over 47 degrees of 

 longitude, and its centre was in transit at l.^h. 34m. 

 Comparing this observation with the position of the same 

 marking at the middle of June, 1901, it will be foujid that 

 it pcrl'orined 1795 rotations in the interval, with a mean 

 period of 91i. 55m. 18-6s. 



But, of course, the most interesting object on Jupiter is 

 the great red spot. This marking exhibited for many 

 years a retardation in motion which increased its rotation 

 period from 9h. 55m. 33-4s. in 1877, to 9h. 55m. 41-Os. in 

 1894. Since the latter year the m^an values hdve been 

 as follow : — 



1895 ... 9 55 41-1 



1896 ... 9 55 41-3 



1897 ... 9 55 41-5 



1898 ... 9 55 41-6 



H. 



8. 



1899 ... 9 55 41-7 



1900 ... 9 55 41-7 



1901 ... 9 55 40-6 



1902 ... 9 55 390 



The spot now follows the zero meridian of Mr. 

 Crommelin's System II. by 52 minutes = 31°-4 of 

 longitude. The observations obtained at Bristol since 

 Jupiter became a morning star are as under : — 



The earlier observations seem to place the spot too 

 far W., but at the time they were made the planet was 

 small and altitude low, and results obtained under such 

 circumstances are rarely so good as those secured nearer 

 the date of opposition. Quite possibly the motion is again 

 retarded. 



The chart of equatorial spots visible in 1902 shows their 

 various longitudes durmg the last six months of the year. 

 The mean rotation period of 24 white and dark spots 

 was 91i. 50m. 26-7s. The irregularities observable in the 

 projected longitudes suggest that these markings have a 

 curious oscillatory motion, and such is indeed the case, 

 though certainly not to the extent apparent in the 

 drawing. Errors in the observed transit times must be 

 partly responsible for a number of the displacements, and 

 considerable changes occur in the dimensions and visible 

 aspect of the spots which must naturally influence the 

 results. But apart from these disturbing causes, the 

 markings have undoubtedly a to-and-fro movement in 

 longitude which sufficiently proves that the equatorial 

 current is subject to decided sectional fluctuations. 



A discussion has recently taken place in Popular 

 Astronoinij as to the relative value of transits of spots 

 obtained by micrometric measurement and by simple eye 

 estimation. There is really a very trifling difference in 

 the accuracy of the two methods. The micrometer seems 

 capable of furni.shing results a little more exact than those 

 depending solely on the eye, but the difference is so small 

 that it may be practically disregarded. Eye-estimated 

 transits are often very precise, for the rapid rotation of 

 Jupiter palpably displaces objects near the central 

 meridian in two or three minutes. Two, or even three 

 minutes of error in the transit time of a spot possess 

 little significance, since the rotation periods are often 

 derived for intervals embracing more than three hundred 

 rotations, and the error will only amount to the small 

 fraction of a second. Even the spots themselves vary 

 in their periods to the extent of tea or twenty seconds 

 or more ! What is of far greater importance in studying 

 the markings is to make a large number of observations 

 so as to be prnrtimHij certain of their individuaj itlentifica- 

 fions. and to follow tliem over as long a period as possible, 

 so that the observational errors may be minified, if not 

 eliminated, by their division among a large number of 

 rotations. The misideutification of markings, and the 

 shortness of the perio<l over wliich they have Iteeu 

 observed, have furnished fruitful sources of error. 



As to the surface currents, whatever may be said in 

 opposition to the plan of sorting them into zones or 

 parallels of latitude, and ascribing mean rates of motion 



* Estimate doubtful, the spot being some distance west of the 



meridian. 



