302 PKOCEEDINGS OF THE AMERICAN ACADEMY 



out the year 1877 as late as Dec. 2, when the observations were 

 abandoned, owing to the proximity of Jupiter to the Sun. In 1878, 

 only a few scattered observations were made from the 10th of February 

 to the end of August, the work being resumed regularly in September 

 only. On the 10th, a step-like marking, resembling that observed the 

 year before, was seen on the southern border of the equatorial belt, 

 apparently at the same place which that mark occupied. It was prob- 

 ably the same object, as seems to result from the calculation of its 

 period of rotation, and from its position in regard to the mark D, which 

 will be next described. 



In the following table are given the dates and times of the passages 

 of the mark C on the central meridian. Similar tables will be given 

 for the other markings described in this paper. The passages of the 

 markings or spots on the central meridian have not been obtained by 

 micrometrical measurements, but are simple eye estimations. This, of 

 course, renders them liable to errors, which cannot, however, exceed 

 two minutes in most cases, I think. 



The passages tabulated have not all been obtained by direct tele- 

 scopic observations in the way described just now, about one third 

 only having thus been observed. The others were obtained after- 

 wards from the drawings themselves, the marking being reduced to 

 the meridian from its position on the drawing, either east or west of 

 the centre. These passages, of course, are liable to still larger errors 

 than the former, the chances for error becoming greater as the distance 

 from the centre increases. For this reason, the position of the 

 markings at the time of observation, that is to say, their distance in 

 time east or west of the central meridian, is given after eacli passage 

 thus obtained ; the symbol -\- indicating that it had passed the merid- 

 ian, and was west of it by the time given when tlie observation was 

 made ; and the symbol — indicating that it was east of it, and had 

 not yet reached the meridian, which it would cross only after the time 

 given had elapsed. The passages on the central meridian which were 

 obtained by direct telescopic observation in the manner above de- 

 scribed are distinguished from the others by the words " On meridian " 

 placed in the third column. 



The time given throughout this paper is the local time, or Cam- 

 bridge mean time. The above remarks apply to all observations and 

 markings tabulated, as well as the arrangement here adopted. 



On July 25 some changes were observed in the mark C, also on 

 Sept. 16 and 21, when the changes were most striking; possibly a 

 jump may be found on these dates. 



