THE ROTATION OF JUPITER. 197 



Dividing the observations of each spot into two groups, 

 finding the mean epoch for each group, and dividing the 

 interval between the epochs by the number of included 

 revolutions, we obtain the following values of the mean 

 period of rotation : 



h. m. s. s. 



From Spot A, 9 55 46*086 + 0*439 



B, 9 SS 44'82i± 0-578 



C, 9 SS 39'24o± 1-650 



E, 9 55 37-812 ±0-636 



F, 9 SS 39-858 +1-650 



Between the zyth of October 1858 and the x^th of 

 January 1859, Sir William Keith Murray Bart,, of Ochter- 

 tyre, made a very interesting series of observations of 

 Jupiter with a fine Munich achromatic refractor of 6^ 

 inches aperture, and he kindly forwarded to me exact 

 copies of a series of diagrams, showing the positions of 

 some of the spots; and also extracts from his journal. 

 These observations were, I believe, made without any 

 special reference to the question of the period of rotation, 

 the observer's principal object being to record and deli- 

 neate any remarkable or interesting phenomena which 

 might present themselves on the disc of the planet ; but 

 as great care has e\'idently been taken in laying down very 

 exactly the positions of the spots in the diagrams, and in 

 noting the corresponding times, I have reduced and dis- 

 cussed the observations of spots A and B, and have 

 obtained the following mean periods : 



h. m. s. s. 



From Spot A, 9 55 45-96 +1-33 

 ■B, 9 55 43-62 + 1-65 



The close agreement between these results and those 

 derived from my own observations will serve to show the 

 degree of reliance which may be placed upon observa- 

 tions of this nature. Had Sir William Keith Murray's 

 observations been continued a month or two longer there 

 is uo doubt the agreement would have been still closer; 

 but unfortunately they were interrupted in consequence of 



