Januaev 16, 1903.] 



SCIENCE. 



85 



their movements to the prevalence of winds 

 on its surface which blow periodically in 

 the same direction." 



Lardner, in his ' Astronomy, ' says : ' In 

 a month or two the whole aspect of the disk 

 may be changed. ' 



In my annual report to the Chicago As- 

 tronomical Society for the year 1881, I 

 stated that the phenomenon seen on the 

 surface of Jupiter was of a more permanent 

 character than had hitherto been believed 

 to be the case. 



In 1878 a large and conspicuous object 

 known as the Great Red Spot was seen on 

 the disk of Jupiter. It appears that this 

 object was first noted on June 2, by Lohse, 

 of Potsdam, but in looking up previous 

 records, we find a spot seen in the same 

 locality by the ancient astronomers. In the 

 years 1664-6, a great red spot was observed 

 by Hook and Cassini. It was situated one 

 third of the semi-diameter of the planet 

 south of the equator in latitvide 6". Its 

 diameter was about one tenth the diameter 

 of Jupiter, or about 8,000 miles. This spot 

 appeared and vanished eight times between 

 the years 1665 and 1708. From 1708 to 

 1713 it was invisible ; the longest time of its 

 continuing to be visible was three years, 

 and the longest period of its disappearing 

 was five years. Since its appearance in 

 1878 it has been visible with large tele- 

 scopes during the whole period, but at times 

 so faint that, except for the indentation in 

 the equatorial belt, the spot, perhaps, would 

 have been lost to astronomers, as it was 

 formerly when they had smaller instru- 

 ments. 



The great red spot is 11.61" or 37.2 de- 

 grees in length, and 8.87" in breadth, or 

 about 27,000 miles long, 9,000 miles broad, 

 elliptical in outline, and, if we suppose the 

 depth of the spot equal to its width, its vol- 

 ume would be about three times that of the 

 earth. This object, which seems to have 



great permanency, is not stationary in 

 either longitude or latitude. 



It was visible in 1869 and 1870, when it 

 was observed by Gledhill on four nights 

 from November 14 to January 25, and on 

 one night by Mayer. The data for ascer- 

 taining the rotation period have been de- 

 rived from the drawings made, and neces- 

 sarily are approximate. 



The rotation period was 9^ 55™ 25.8^, or 

 about eight seconds less than it was in 1879. 

 From the observations made in 1878 I 

 derived a rotation period of 9*^ 55°^ 33.7^. 

 Since the rotation period had been in- 

 creasing for twenty years, the observations 

 in 1869 are of value in tracing the motions 

 of this object. 



I may add that' Mr. W. F. Denning, who 

 has compiled the observations of what is 

 presumed to be the red spots from 1831 to 

 1899, finds a rotation period of 9"^ 55™ 34^ 

 between 1869 and 1878, by assuming the 

 number of -rotations between consecutive 

 observations. But where the interval is 

 five years and upwards this is a very un- 

 safe method of procedure, as Avill be per- 

 ceived from the motions which have been 

 studied during the last twenty-three years. 



From the measures which I have made 

 every year I have determined the rotation 

 period for the red spot from 1879 up to the 

 present time, and with the minimum value 

 in 1879 of 9^ SS-" 34^ The diagram shows 

 the rotation period at any point between 

 1879 and the present time. The vertical 

 lines are intervals of 400 days, one day 

 more than the synodic period of the planet. 

 The horizontal lines represent seconds of 

 arc, so that the rotation period at any point 

 will be shown on the curve, the seconds be- 

 ing at the left hand of the diagram, and the 

 time at the bottom of the diagram. The 

 rotations for this curve were computed for 

 intervals of 400 days by using at each 

 epoch about twelve normal places, and the 



