300 PEOCEEDINGS OF THE AMERICAN ACADEMY 



after having accomplished five rotations, the mean of which equals 

 gh ^jm Qs^ 'pjjQ position of this spot was in the northern part of the 

 equatorial belt, extending as far as its border on that side. For 

 convenience we will call this marking A : the other markings to be 

 described will be similarly designated by a letter of the alphabet. 



Mark B. 



In contrast with the usual behavior of the spots seen on the surface 

 of Jupiter in years of calm, and also to show how much these spots 

 are to be relied upon to determine the period of rotation of the 

 planet, I will give a brief account of an observation made in 1S76, 

 from which an idea can be formed of the rapidity with which changes 

 sometimes take place on Jupiter, and of the swiftness which some- 

 times animates these spots. 



On May 25, Jupiter was observed and a drawing made at 8^ 37". 

 At this time, and at least five minutes later, nothing unusual or re- 

 markable was to be seen on the planet. At 9*^ 4", however, an 

 angular marking resting on the outside border of the equatorial belt, 

 on its south side, was visible near the east limb. This mark advanced 

 rapidly towards the centre of the disk, which it had passed at 9^ 32°, 

 when a third drawing was made. At 9*^ 50™ the angular marking had 

 reached the western limb, having then crossed the disk of Jupiter from 

 east to west in less than an hour's time. 



The angular mark of which the motion has just been described was 

 formed by an oblique dark band, resting on its preceding side on the 

 xapper band, forming the southern margin of the equatorial belt. As 

 the angular mark advanced on the disk, the space behind it enlarged 

 considerably, the upper band limiting it southwards, keeping the same 

 obliquity with the equatorial belt as far as it could be seen on the east 

 limb. At 9^^ 32° the oblique band on the east limb had a latitude of 

 50° south. After the angular mark had crossed the western limb, it 

 continued its tremendous onward motion, as could be seen by the con- 

 stantly increasing latitude of the upper band on the west limb, and at 

 lO*" 20° this upper band was parallel with the southern border of the 

 equatorial belt. The space then comprised between this upper band 

 and the southern margin of the equatorial belt was equal to 35°, and 

 was about twice as broad as the latter belt before the disturbance 

 occurred. This space appeared tinted with a delicate pink color, 

 intermingled with white cumulus-like spots resembling exactly those 

 usually seen on the equatorial belt. When the upper band reached 



