1 877.] Movements of Jupiter's Cloud-Masses. 199 
throughout the period mentioned ; but the special peculiarity 
to which attention is asked is that during an interval of five 
days they remained in the same relative position without 
any variation whatever. Their stability in respeCt of lati- 
tude during those five days is undoubted ; but the question 
is whether or not they were equally stable in longitude. 
This remark only applies to the first five days of the series, 
because at the end of twelve days a certain deviation was 
obvious. The distance between the two spots occupied 
about 42 degrees of Jovian longitude, or about 33,000 miles. 
Their diameter is nearly equal, being estimated at about 
one-fourteenth of the planet’s diameter, or 6310 miles.” 
The interval of time between these first two observations 
“was 119 hours, — that is to say, twelve rotations of the 
planet according to Airy’s determination, during which time 
their distance apart and their latitude remained constant.” 
Between the first and second observations the two spots had 
gained “44 m. 6 s. in time. Assuming Airy’s rotation, 
viz., 9 h. 55 m. 21 s., the spots have gained on the planet’s 
surface at the rate of 4 m. 2 s. in each revolution.” 
Between the second observation and the third “ there was 
an interval of seven days, or seventeen rotations of the 
planet ; and the same two spots turn up again somewhat 
earlier than the calculated time. It unfortunately happens,” 
proceeds Mr. Brett, “that on this occasion their configuration 
had undergone some change ; but their dimensions and the 
distance between them remain very much as before. The 
most important circumstance respecting them is, that their 
rate of progress shows a certain acceleration.” The change, 
however, in these seven days, is not such as to permit us to 
believe that the same pair of spots was under observation. 
If so, a change in latitude much more remarkable than the 
change in longitude had taken place ; for the one which was 
the most northerly by about 6000 miles at the beginning of 
the seven days was the most southerly by nearly the same 
amount at the end of that period. Considering that in the 
five days between the first and second observations no 
change of latitude took place, it may fairly be doubted 
whether a change of the kind and so rapid — amounting, in 
faCt, to nearly 900 miles per day, could have taken place in 
the interval. Proper motions in latitude may indeed be 
regarded as not less likely to occur in the case of Jupiter 
than in that of the sun, where they certainly sometimes 
occur; but all the observations hitherto made on Jupiter 
assure us that, in his case as in the sun’s, proper motions in 
latitude would be very much slower than proper motions in 
