SCIENCE. 



[N. S. Vol. XVII. No. 420. 



claanges in this belt one may arrive at some 

 idea regarding motions taking place on the 

 surfacfe of the planet. The systematic de- 

 termination of motion in latitude has never 

 been undertaken hy any one previous to the 

 observations which I began in 1879. Occa- 

 sionally latitudes have been measured dur- 

 ing one opposition. Ai-ago, in 'Astronome 

 Populaire, ' raised the question whether 

 the belts on Jupiter are fixed in size and 

 position, and he gives some measures of 

 the positions from 1811 to 1837, and takes 

 the mean of these various measures for get- 

 ting the mean position of the belts on the 

 planet. These observations are approxi- 

 mate, and are used without regard to the 

 position of the earth above and below 

 Jupiter's equator. From 1879 to the 

 present time the latitude and width of 

 the great equatorial belt have been meas- 

 ured on nearly every observing night, 

 so that we may ascertain the position 

 of the edge of the belt at any instant. 

 It is found that the north edge of the 

 belt has had a drift in latitude of nearly 

 4" of arc or 12 degrees, and the south 

 edge about the same amount. The changes 

 in the drift of the belt are usually slow 

 and gradual, but it is possible sometimes 

 that considerable change may be observed 

 in the course of a few days. The dia- 

 gram indicates the position of the edge 

 of the belt from 1879 to 1902, and it is 

 of very great interest in showing at a 

 glance the changes that have taken place 

 in latitude. From the study of this 

 diagram it appears that the disturbances 

 take place on both edges of the belt at 

 practically the same time. The matter 

 composing the belts generally has a motion 

 on both sides of the equator in opposite 

 directions. 



In 1879 the whole width of the belt was 

 about 7" of arc. In 1882 it widened out 

 and has at times reached a width of about 



13" of arc. The edges of the belt remain 

 practically parallel to the equator in all 

 longitudes. I have noticed two marked ex- 

 ceptions. On October 3, 1882, there was a 

 curved projection in longitude plus 30 min- 

 utes, following the great red spot. On 

 October 14 the edge was smooth at the same 

 longitude and the whole belt had drifted 

 so far north as to coalesce with B^. Also 

 on February 24, 1897, in longitude plus 

 -five hours, the preceding half of the north 

 edge of the belt drifted about two seconds 

 farther north than the following portion. 

 On February 27, however, the edge of the 

 belt was comparatively smooth in the same 

 longitude. 



Aside from the drift of the edges of the 

 belt in latitude, the belt itself changes 

 dimensions from time to time to a consid- 

 erable extent, and these changes have been 

 studied from micrometrical measurements 

 since 1895. The diagram shows the width 

 of the two portions of the equatorial belt 

 at any instant from 1895 to 1902. The 

 diagram indicates the width and not the 

 shape of the belt at any time. Now it is 

 seen, taking the portion of the belt north of 

 the equator, at times it becomes very 

 narrow; for instance in 1896 it was about 

 1" arc in width, 1897 it was about 5" in 

 width, and then it became narrower again 

 in 1898, and continued wide fi*om that time 

 until 1901, when it was less than 1" are in 

 width and appeared as a faint line on the 

 planet. The south portion of the belt has 

 not passed through so great change during 

 the five years, and has been more steady in 

 latitude and width. On either side of the 

 equator are fainter belts which usually ex- 

 tend to 40 degrees of latitude as separate 

 belts. These faint belts are subject to 

 change, in both size and position, from 

 year to year. 



On the belts and on the surface of the 

 planet there are frequently seen small 



