320 PROCEEDINGS OF THE AMERICAN ACADEMY 



according to the velocity and direction of motion of the mark we take 

 for reference iu our observations ; this is all. 



But if it is impossible to obtain the true period of rotation of Jupiter 

 in the present condition of the planet, it is possible to approach it, 

 and to come within narrow limits of the true period, by the observa- 

 tion of a great number of markings, having great permanency, and 

 lasting unchanged for a long period, like the markings C, D, and E. 

 It seems probable that this last, the great red spot, gives us, about 

 as nearly as we ever can obtain it, the period of rotation of Jupiter ; 

 although the period of this spot is far from being perfectly uniform, as 

 it is certainly endowed with a motion of its own, which becomes very 

 perceptible on some days, — a fact which is easily established from the 

 changes of form of that spot described above. It is remarkable that 

 the period of the red spot, greater at first, has decreased afterwards, 

 and then increased at last so as to be very nearly equal to what it was 

 at first. The mean period of this spot from Sept. 25 to Dec. 28, 1878, 

 equals 9'' oo"" 43.61"; from Aug. 9 to Nov. 9, 1880, it equals 9^^ oo" 

 30.8 IS and from Nov. 9, 1880, to Feb. 13, 1881, it equals 9'' 55'^ 43.96'. 

 The mean intermediate period was then very nearly 13' shorter than 

 were the fii'st and last. 



It is remarkable that of all the markings observed the red spot gives 

 the greatest period of rotation to Jupiter; next come the marks D and 

 C, which also, like the red spot, had a long duration, and were on the 

 south side and very nearly in the same latitude as this spot. These 

 observations do not support the assumption, however, that the proper 

 motion of the spots follows any law in regard to latitude, as is sup- 

 posed to be the case with sun-spots ; for the spots G, which occupied 

 about the same latitude iu the north as tlie red spot occupied in the 

 south, had a period over 8™ shorter than has this last. The markings 

 A, F, H, and I, although very nearly on the same parallel of south 

 latitude, had different periods, the extreme of which differed by 

 nearly 6™. The oval spot K, although in higher southern latitudes 

 than the red spot, yet had a shorter period of rotation, and we have 

 shown that the angular mark A, of 1876, observed on the southern 

 border of the equatorial belt, had a period of 2*^ only, while all the 

 markings of that hemisphere extending up to the pole had very nearly 

 the same rapid proper motion. From the behavior of F, G, and II, it 

 would seem that, all being equal, the period of rotation is shorter when 

 the spot is new than when it has lasted for some time. 



I have shown that the surface of Jupiter, like that of the Sun, has 

 years of calm and years of great disturbance. In the year 1876, the 



