THE ROTATION OF JUPITER. 195 



The object of each night's observations being to deter- 

 mine, as accurately as possible, the time when the meridian 

 of a spot passed through the centre of the disc of the 

 planet, careful eye estimations of the position of the spot 

 were taken at intervals during its transit across the disc ; 

 each result was then reduced to the central position by 

 the aid of small tables constructed for the purpose ; and 

 the mean of all the reduced results — allowing weights 

 in certain cases — was taken as the true time of the 

 spot's apparent meridional conjunction with the centre 

 of the disc. The times thus obtained were afterwards 

 reduced to the epoch of the first observation, November 

 yth, 10 A. 40m. G. M. T., by applying the corrections due 

 to the altered geocentric longitude of Jupiter, and to the 

 changes in the value of the light equation. 



Attempts were made to determine the positions of the 

 spots by micrometrical measurement ; but it was found 

 that the results obtained in this way did not exceed in 

 accuracy (except in the case of a very conspicuous spot 

 and under highly favourable atmospherical circumstances) 

 those obtained from eye estimations, or from carefully 

 drawn diagrams; and, moreover, valuable estimations 

 coald often be made when, from frequent interruptions by 

 rapidly passing clouds, complete micrometrical measure- 

 ments were quite impracticable. 



The number of spots observed was seven, and they were 

 denoted in the journal by the letters A, B, C, &c., in the 

 order in which they were first seen ; but the observations 

 of five only were sufiiciently numerous and exact to be 

 available for the determination of their periods of rotation 

 They were all distributed in a narrow band or zone in the 

 southern hemisphere, six of them being nearly on the 

 same parallel of latitude, and the seventh only about fifteen 

 degrees more to the south. It will be evident that this 

 peculiarity of distribution was very unfavourable for an 



