Janiaby, 1904.] 



KNOWLEDGE. 



With one or two exceptinns these surface cuvrciits are 

 pretty constaut. Their velooily varies within eertaiu 

 limits, and the hititude of their liouiichiiies is not always 

 the same, but whenever detinite sj.H>ts or observable 

 oondeusations appear their niovenieuts of rotation are 

 nearly always found to conform more or less closely to 

 the normal s]>eed of that latitude. In an article in the 

 February. 1;10:5, uuuiber of PopnJar AstniiiDitiii, Prof. 

 G. W. Houijh questions the existence of several of these 

 surface currents. Consideriufr. however, the great mass 

 of existing: evidence. I venture to thiuli that his conclusion 

 is altogether unfounded, and that the reality of the 

 currents is lieyond dispute. In January, 189ti, a valuable 

 pajier bv Mr. A. S. WilUanis was published in the 

 Monthly' Notlceg, E. A. S., "On the Drift of the Surface 

 Material of Jupiter in Different Latitudes." In that 

 paper Mr. Williams brought together the results of 

 numerous eminent observers in various years, and gave a 

 cleaj" account of nine separate and distinct ctirrents. It is 

 worthy of m>te that the .arrangement of these currents, 

 unlike those of the sun, is by no means symmetrical, neither 

 is that of the two hemispheres the same. Moreover, the 

 N. hemisphere contains in contiguity the swiftest and the 

 slowest that have yet been observed. 



The following table shows the general arrangement of 

 these surface currents, but it must be understood that. 

 both the limiting latitudes and the rotation periods are 

 subject to certain variations : — 



No. 



Latitude. 



Rotation 

 Period. 



Kemarka. 



1 I +80° to +31° 



2 I +J4'' to +2*° 



b. m. s. 



9 55 37-5 



f9 ot .30 ) 



1 9 56 30 ( 



From Polar Eesious to N.N. Temp. Iielt. 

 From N.N. Temi). belt toN. uoiiiponent 

 of N. Temi). holt. 



These cun-ents must be discussed more in detail. 



No. 1. — There appears to be some uncertainty as to how 

 far north this current extends. In 1888 and again in 18!<2, 

 Mr. Williams observed dark streaks which extended into 

 very high N. latitudes and moved in accordance with the 

 tabulated velocity. Since then Captain P. B. Mt>lesw()rtli, 

 who has made quite a unique series of .Jovian observations 

 under very fine seeing conditions in Ceylon, Las succeeded 

 in detecting a number of light and dark spots and streaks 

 in the Polar regions. Amongst these, he found in I!t(»l five 

 dusky streaks in about latitude •jO"', which gave a mean 

 period of 9b. .56m. 37s. It is cjuite jjrobal)Ie that tb(> 

 surface drift in these regions may be variable from year to 

 year. More observations are much needed to settle tiie 

 question of the minor, and at present doubtful, currents on 

 the surface of Jupiter. 



No. 2. — The drift in this region is not constant. At 

 times when the N. hemisjjhere is in a state of disturbance 

 spots are liable to appear which have a decidedly rajiid 

 rate of motion. As a general rule, however, it is found 

 that markings in this neighbourhood exhibit the slowest 

 movement of any on the disc. 



No. 3. — This is unquestionably the most remarkable, as 



it is the swiftest, of all the .Jovian currents. Our know- 

 ledge of it has been well summavized by Mr. \V. V. 

 Denning in a ]>aper eutitleil " On a i'l-obabie Instance of 

 Perioilically Kccuirent Disturbance on the Surface of 

 Jupiter," published in huiithlij Notices, K. A. S., Dccemlior, 

 1898. It ajipears that at intervals of little more than ten 

 years spots hav<' fi-ecpiently ajipcared on the S. side of the N, 

 temju'rate belt which have exhibited a velocity whicii is 

 extraordinary. As already pointed out, this swift current 

 exists si<h^ by sidt^i with the slowest of the disc (No. 2), and 

 taking their extreme v.ilues the dii3'erence of velocity 

 amounts to about ^t!.') miles per hour. It was thought 

 that another outbreak of these rapidly-moving spots would 

 occur at the end of 1900 or beginning of 1901. Unfor- 

 tunately no such occurrence was observed, but it is quite 

 possiljle that spots may have appeared and escaped 

 tietection, as in December, 1900, the planet was in con- 

 junction with the sun. 



No. 4. — This— commonly known as the N. Tropical 

 Current — is another of the most important of the Jovian 

 currents, and is generally in evidence. In some years 

 when theN.e(|uatorial l)elt is narrow, a number of dark s])ots 

 are seen quite detached from this belt (see Figs. 1 and :5), 

 and in 189S and 190;! these were connected by a fine narrow 

 line like beads strung on a thread. This narrow line is shown 

 in Fig. 3 starting from one of these spots. On other 

 occasions the N. etpiatorial belt extends so far north as to 

 include this region, but it is found that the spots at its 

 edge, which are often very numerous and detinite, conform 

 to the normal velocity of the N. Tropical Current, even 

 though the S. edge of the belt l)e drifting at the same 

 rate as the equatorial zone. I have given 9h. .5.5m. :12s. 

 as the rotation period of this region, but it frecjuently 

 happens that spots exhibit a period very cousideralily 

 longer than this, and also very considerably shorter. A 

 remarkable diversity of speed was apparent in this 

 current in 1899. In that year I received a large number 

 of transit observations of N. tropical spots from several 

 observers, so that an ample amount of material was 

 available for discussion. 1 had previously found from 

 observations secured comjiaratively early in the apparition 

 that a dark spot — shown in Fig. 4 lyiug in a distinct bay 

 on the N. sidi^ of the N. equatorial belt — was moving at an 

 altogether abnormal rate, but when the whole of the 

 materials to h.and were charted and examined, it was 

 found that the spots between longitudes 140" and 260'^ 

 had a mean rolatiou period of 18'.5 seconds less than that 

 of the remainder of the current. The exact values were 

 91i. .5.5ni. 1.5'3s. and 9h. 5.5m. 3;J'9s. respectively. So far 

 as I am aware so rapid a drift as that indicated by the 

 former value has never been observed in this latitude 

 before. Further, it w'as noticed that the limits of longitude 

 which ineludeil this swift rotation were constant during 

 the [leriod covered by the observations. Spots starting 

 from X 2<)0° quickly hurried forward, and rapidly-moving 

 spots on arriving at A 140" suddenly slowed down. A full 

 account of this remarkable disturbance will be found in 

 inv paper on "The Extra-Equatorial Currents of Jupiter 

 in 1899," jiublished in Monthli/ Notices, R. A. S., January, 

 1900. 



No. .5. — We now come to the Great Equatorial Current. 

 The northern boundary of this current is variable. When 

 narrow the whole of the N. equatorial belt ajipears to be 

 included within its limits, but at other times only the 

 southern component, or possilily the whole of the belt may 

 lie without it. At any rate, spots near the N. limit of 

 the /.one frequently exhibit a period a few seconds longer 

 than those at or near the N. edge of the S. equatorial 

 belt. It is to these latter that most of the determinations 

 of velocity in previous years refer. It is worth noticing 



