32 
NATURE 
[May 14, 1885 
spot has now been followed since 1878, and though 
apparently on the verge of absolute extinction, it may yet 
linger on a considerable time in its present feeble aspect 
until possibly it is again enabled to obtrude upon general 
notice as an object of great prominence. It may not 
return under precisely the same outline as formerly, or 
exhibit the same depth of tone or degree of colouring, 
for, doubtless, some new development is to be anti- 
cipated on this disturbed region of the planet. In case 
of any distinct reappearance it will be important to deter- 
mine that it occurred from the exact position so long 
tenanted by the old spot. The motion of this feature has 
been so thoroughly followed during the last seven years, 
that it will be feasible to compute its predicted place with 
great nicety in future months. In the mean time, and 
until the spot finally withdraws from reach, the same 
necessity exists as before of recording the times of its 
passages across the central meridian of Jupiter. And 
even assuming the total extinction of the spot, and that 
its place immediately south of the great equatorial south 
belt should resume the unbroken zonal arrangement exist- 
ing in other longitudes, it will be necessary to re-examine 
this region occasionally for traces of any subsequent out- 
break from the same focus. 
During the last three years this object has given a rota- 
tion period of gh. 55m. 39"1s., which has been steadily 
maintained throughout each opposition, subject to some 
minor disturbances partly due to errors of observation. 
The first few years of its existence it showed an increasing 
retardation of motion, which lengthened the period from 
gh. 55m. 34s. to that already quoted, but, contemporarily 
with the decay of the spot in 1882, the velocity ceased to 
slacken, and the results accumulated during the past few 
oppositions prove it to have been equable in a marked 
degree. 
With reference to the equatorial white spot some striking 
phenomena have been presented during the past winter. 
Between October 4, 1884, and January 13, 1885, its motion 
appears to have increased in an alarming ratio. The spot 
continued to rush on far in advance of its computed 
places, and all the while exhibited a more brilliant appear- 
ance than at any preceding epoch since the autumn of 
1880, when it first came under systematic observation. 
The form and appearance of the spot have been so special 
as to prevent any confusion in mistaking it for other white 
spots in nearly the same latitude.. Between October 4 and 
January 13,1885, the rotation period was gh. 49m. 51°95s., 
but the great increase in velocity evidently occurred to- 
wards the end of November. Between November 21, 1884, 
and January 13, 1885, the period was only gh. 49m. 38°45s., 
or 34 seconds less than the mean period of gh. 50m. 12'25s. 
shown by the same spot during the two preceding years. 
When the first intimation of this great increase of speed 
forced itself upon my notice, I at once resolved to obtain 
as many observations as possible, in order to assure 
myself more certainly of the fact. Much cloudy, wet 
weather ensued, but I observed the spot on fourteen 
occasions between November 27 and January 13. A 
lengthened period of overcast skies then supervened, and 
I saw ncthing more of Jupiter until January 27, when the 
place of the spot, computed on the basis of my prior 
observations, appeared absolutely vacant. About 15° E. 
there was, however, a remarkably brilliant spot, the exact 
counterpart of the one previously observed. Then arose 
the question of identity. Could the velocity have become 
so much retarded in the fortnight’s interval from January 
13 to 27 as to have occasioned so considerable a displace- 
ment in longitude? From my observation on January 
13 and several preceding nights the spot had shown an 
increasing disposition to slacken, and, from records 
obtained in previous years, the motion was known to 
fluctuate in the most unaccountable manner. In the 
seventeen days from September 30 to October 17, 1881, 
I noted the spot underwent a sudden translation of 11°°6 
in the direction of east longitude. The fact was inde- 
pendently confirmed by Prof. Hough at Chicago and Mr. 
Stanley Williams at Brighton. The most obvious de- 
partures from the mean rate of motion have been detected 
in other instances, and I am therefore led to conclude 
that the objects observed on January 13 and 27, 1885, 
were, notwithstanding their discordance of position, really 
identical objects. The consistent brilliancy of the marking 
alluded to, for several months before the cloudy period 
set in, is entirely opposed to the idea that it could have 
suddenly disappeared. And the real displacement is not 
so large as the limiting observations suggest. Deriving 
a mean from my results near January 13 and 27, I obtain 
the following figures :— 
Spot precedes 
Long. 
88 d 2 
1555 Ist ae lan (878 34) 
Jan. 7 to 13, mean of 7 obs. ... ... 64'0 321°0 
Jan. 27 to Feb. 6, mean of 6 obs. ... 464 ... 331°4 
Adopting this mean, we practically eliminate errors in 
single observations, and in the present case it is fortunate” 
I obtained so many transits just before and after the 
period of cloud. The real displacement is seen from this 
comparison to be only 10°'7, which is quite within the 
limits of previous experience. And if the fact of identity 
had not been rendered a very tenable hypothesis by past 
observation, I should have regarded the brilliant appear- 
ance of the spot and its comparative isolation as con- 
clusive. Moreover, during the period that this object 
continued moving so rapidly, I often carefully examined 
the place where, had no change occurred, it must have 
been presented, but no object having a remote like- 
ness to the old spot could bedetected. Having observed 
this feature on the central meridian on more than 200 
nights, I am familiar with its usual aspect, and could not 
possibly have overlooked it, on the many occasions when 
I looked for it in vain, had the spot retained the approxi- 
mate place assigned to it from the observations of pre- 
ceding years. 
Let us now analyse the degree and period of the 
remarkable velocity alluded to. Arranging my observa- 
tions into short intervals, the following are the rotation- 
periods severally derived from them :— 
Spot Spot 
Interval gained gained Number. 
1884 in cn in of Period 
Minutes 1st.Mer. Long. Rotations, 
m. 4 h. ms. 
Oct. 4 to Nov. 7 2 4Bj985; weer 685 ieee) 1400) ase, (BB Etec ESOS 
Nov. 7 to Nov. 21 ... 20,064 . (3°95 «0» 2°I «se 34 <=» 9 50 6/07 
Nov. 21 to Nov. 27 ... 8838 .. I5'l we QO2 I5 «= 9 49 11°85 
Noy. 27 to Dec. gw. 17,098 «+ 1I7°4 «te TOO os 29 a 9 49 36°25 
Dec. 9 to Dec. 18 .. 12,970 » 1378 « 8'5 22 o- 9 49 34°61 
Dec. 18 to Dec. 24 «.. 8843 «. §=69"3 ws 36. T5 se 9 49 35°05 
Dec. 24 to Dec. 31 ... 10,023 95 ++ 518 we 17 oe 9 49 30°72 
Dec, 31 to Jan.8 «. =11,208 46 - 28 ww. 19 oe 9 49 57°73 
1885 
Jan. § to Jan. 13 7078 3°5 2°2 ee I2 oe 9 49 54°75 
Lost Lost 
Jan. 13 to Jan. 27 20,089 «+» 24°9  35°2 «= 34 we Q 50 50°19 
Jan. 27 to April 19 ... 118,042 97 5°7 «© 200 = Q 50 X5:00 
The period of really great acceleration extended over 
forty days (November 21 to December 31), and it is 
remarkable that in the mean time the spot had completed 
exactly one revolution of Jupiter relatively to the red spot. 
In fact, the sudden increase and diminution of velocity 
occurred with the white spot following the red about 
2h. 44m., so that there was a difference of 100° in the 
longitude. The maximum speed appears to have been 
shown between November 21 and 27, when the rotation- 
period was one minute less than the mean of the two 
preceding years. But my observation of November 21 
was considered rather late, and the interval being a very 
short one of only six days, would originate a rather large 
error. But the four short intervals, from November 27 to 
December 31, exhibit a singular consistency in the re- 
sulting periods, the mean being 9h. 49m. 36°16s., which 
proves the real increase of speed to have been 36’09s. in 
