224 
It has now been visible for a period exceeding three 
years, and its conspicuous and decided aspect as it 
nightly crosses the central meridian of Jupiter, suffi- 
ciently predicates that its existence is likely to be pro- 
longed a considerable time yet. The spot is elliptical in 
form with tapering ends; it occupies about fifty-five 
minutes in its entire transit over the centre of the illumi- 
nated dise. No distinct alteration in its appearance has 
been recorded during the last two years. Minor changes 
have probably occurred, though too minute to be appre- 
ciable, for when we consider the perpetual state of com- 
motion under which the other. markings exist, we cannot 
regard this particuiar object as absolutely free from 
similar influences, though they may have hitherto eluded 
detection. One of the most recent measures of the spot 
give it a length of 29,000 miles, and a breadth of 8,300 
so that the length is to the breadth as 7 to 2. 
The telescopic history of this planet contains many 
instances of fairly persistent spots having been observed 
and utilised as a ready means of determining the period 
great red spot in ‘J ransit, 
spot. 
Fic. 1.—The 
culty in adopting a period to satisfy the observations, and 
there is no doubt that similar independent movements in 
the objects observed were the c of the differences in 
the periods assigned by various former authorities for the 
rotation which has varied from gh. to gh. 56m., and 
in truth it is doubtful whether we ever obtain the 
means of deriving the value with conclusive satisfaction. 
We can of course compute with nicety the periods of 
individual markings , but different ones have different 
periods. It is doubtful whether we can select the mark- 
ings which are representativ of the real period, for the 
spots are evidently influenced by a series of atmospheric 
currents which displac ce their relative positions on the 
disc in a very short interval of time. In the case of the 
great red spot, ithas been surmised from its permanence 
and regularity of apparition that it very nearly represents 
the true rotating feriod of the planet's sphere, and it is 
49m. 
NATURE 
[ Fan. 5, 1882 
of the planet’s rotation. But the records of former years 
can furnish no parallel to the extent and accuracy of the 
modern observations. The old observers were in a 
measure isolated, and their work often lacked corrobora- 
tion. Circumstances are now changed entirely. Observers 
have become far more numerous, astronomical appliances 
have been greatly improved, and the science has become 
more popular with increased facilities, so that where one 
powerful telescope was found half a century ago, there 
are at least ten at the present day. The result is that all 
attractive phenomena, such as planetary markings, are 
eagerly watched and recorded, so that interesting com- 
parisons and confirmations, impossible in former times, 
are often the natural cutcome. In regard to the large 
spot on Jupiter it is certain that no previous observations 
can vie with it either in completeness or precision, and it 
must obviously supply the data for determining the rota- 
tion period of Jupiter with a degree of reliance beyond all 
parallel. 
Proper motion in the spot itself originates a slight diffi- 
narrow belt with I 
13 Following side of the red 
ight and dark ovals 
etermined 
as 
gratifying to note that this element has been d 
by sev eral observers with excellent corroboration 
follows :— 
Prof. G. W. Ho 
A. ‘aes th ‘ = St 
Juwoie Schmidt ... 9 
H. Pratt... ceca 
VUuiuoma 
Munna % 
Os te en 
2 Gs Ga Gd 
uly to December 
uw 
These values are all within one second of the mean 
=gh. 55m. 34'5s. Several observers have remarked that 
the motion of the spot has slackened somewhat since 
1879. 
Turning now to the equatorial region of Jupiter, we 
shea find here not only a large extent of detail, but 
narked evidences of great and rapid changes. Imme- 
diately north and south of the equator there is a very 
well-defined dark belt, and between these are a series of 
