MOVEMENTS OF JUPITER'S CLOUD-MASSES. 91 



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 cliange ; but their 

 dimensions and the distance between them remain very much as before. 

 The most important circumstance respecting them is, tliat 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 6,000 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 longitude. We must be content with 

 the evidence of jDroper motion afforded by the first five days of ob- 

 servation. (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 tlie 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 accom- 

 plished by each at about 267,000 miles, or, say, 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 1,800 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 derived 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 dif- 

 ferent from the rotation-rate derived from others, and that soms spots 

 (if not all) are certainly not constant in position with respect to the 

 solid nucleus of the planet. That the spots observed by Airy, Mad- 

 ler, and others, should have indicated a slower rate of rotation than 

 those observed by Mr. Brett, may fairly be ascribed to the fact that 

 the former were at some distance from the equator, while the latter 

 were nearly equatorial. For matter thrown up from the equatorial 

 parts of the true surface of the concealed planet would manifestly 



