200 
Movements of Jupiter's Cloud-Masses. 
[April, 
longitude. We must be content with the evidence of proper 
motion afforded by the first five days of observation. (The 
fourth observation only followed the third by about twenty 
minutes.) 
Now, taking this evidence as it stands, and making fair 
allowance for probable error in an observation of the sort, 
we may consider that during the 119 hours the two spots 
were gaining on the estimated rotation-period of the planet 
by about four minutes per rotation. As they both lie on the 
equatorial belt, we may take the circuit accomplished by 
each at about 267,000 miles, or their rate at about 
270,000 in ten hours, or 27,000 miles per hour. Hence the 
distance traversed in four minutes would be about 1800 miles, 
which would be about the gain per rotation. One-tenth of 
this, or 180 miles, would be the hourly gain, as compared 
with the estimated rotation-rate. Mr. Brett takes the least 
proper motion at 165 miles per hour. 
He points out justly that the rotation-rate has been de- 
rived from observations of some such spots. So that in 
reality the only inference we can form is, that the rotation- 
rate derived from some spots is different from the rotation- 
rate derived from others, and that some spots (if not all) 
are certainly not constant in position with respedd to the 
solid nucleus of the planet. That the spots observed by 
Airy, Madler, and others should have indicated a slower 
rate of rotation than those observed by Mr. Brett may fairly 
be ascribed to the fadt that the former were at some distance 
from the equator, while these last were nearly equatorial. 
For matter thrown up from the equatorial parts of the true 
surface of the concealed planet would manifestly differ less 
in velocity from the superincumbent atmosphere into 
which they were driven than would masses expelled from 
higher latitudes. (It is probable that the same explanation 
applies also in the case of the sun.) 
This conclusion, that the spots of Jupiter have rapid rates 
of relative motion, would of itself be of singular interest, 
especially when we remember that the larger white spots 
represent masses of cloud 5000 or 6000 miles in diameter. 
That such masses should be carried along with velocities so 
enormous as to change their positions relatively to each 
other, at a rate sometimes of more than 150 miles per hour, 
is a startling and stupendous fadt. But it appears to me 
that the fadt is still more interesting in what it suggests 
than in what it reveals. The movements taking place in 
the deep atmosphere of Jupiter are very wonderful, but the 
cause of these movements is yet better worthy of study. 
