January 16, 1903.] 



SCIENCE. 



89 



spots, sometimes white and sometimes 

 black, viz., 2,000 miles or more in diam- 

 eter, and from the observations of these 

 spots we have determined the rotation 

 period of the planet for different parts 

 of the surface. The spots, which appear 

 near the north margin of the equatorial 

 belt nearly every opposition and are some- 

 times permanent for two or three years, 

 and have a slight motion in latitude, 

 only a fraction of 1" of arc, whereas 

 the belt may move 3" or more in lati- 

 tude in one year. It seems to me that 

 this fact has an important bearing as to 

 location of the objects, viz., the belt and the 

 spots. I infer from the slight displace- 

 ment of the spots that they lie at a lower 

 level in the Jovian surface than the equa- 

 torial belt, and for the same reason the 

 great red spot lies at a lower level. 



The transits of the satellites of Jupiter 

 offer phenomena which have a direct bear- 

 ing on the constitution of the planet. The 

 satellites at times cross all parts of the disk 

 in transit. For a normal transit the satel- 

 lite disappears at some distance from the 

 disk after ingress and reappears at a simi- 

 • lar distance before egress. From this fact 

 it is concluded that the center of the disk 

 of Jupiter has the same reflecting power as 

 the satellites. With the 18-J-" refractor I 

 have ascertained that a satellite can be fol- 

 lowed for a distance of 10" of arc from the 

 limb or nearly one quarter the diameter of 

 the disk before it disappears in transit. 

 However, when the transit occurs within 

 10" of the north or south limbs, the satel- 

 lite can be seen during the entire transit 

 across the disk. Now since the satellite is 

 not supposed to be hot enough to give light, 

 we conclude there is not sufficient heat in 

 the planet to produce light. The observa- 

 tion of the eclipse of the satellite also 

 shows that it has no inherent light of its 



Aside from the period of 9"^ 55™, some 

 spots and markings give a shorter period of 

 9^ 50"\ indicating that these objects have a 

 motion of about 250 miles an hour in the 

 direction of the planet's rotation, assuming 

 that the true rotation period is 9*^ 55"'. 

 For mechanical reasons the spots which give 

 this shorter period must necessarily be 

 located above the spots which give the 

 longer period of 9"^ 55™. From 1879 to 

 1885 two white spots in latitude 6 degrees 

 south were observed every year, giving a 

 rotation period of 9^" 50™ plus. The white 

 spots, during the last twenty years, which 

 give this short period, have been observed 

 between the latitudes plus 11 and minus 

 8, and also in one year, in 1891, black spots 

 Avhich gave a short period were observed in 

 latitude 20 degrees north. The spots and 

 markings which give the long period of 

 gh 55ra iiave been observed in latitudes be- 

 tween 37 degrees north and 38 degrees 

 south and within 12 degrees of the equator. 



The equatorial belt sometimes approaches 

 the equator very closely, and its rotation 

 for some years has been the same as that 

 of the great red spot, for the spot and the 

 belt have, as we know, maintained the same 

 position toward each other. Hence we find 

 the longer rotation period of 9'^ 55™ in pre- 

 cisely the same latitude as the shorter 

 period. On examining the table of rota- 

 tions there does not seem to be any connec- 

 tion between latitude and rotation period, 

 as has often been alleged. The longest 

 period which I observed, covering an in- 

 terval of 156 days, is 9^^ 56™ 0.4^ which 

 was in latitude 26 degrees north. 



Mr. A. S. Williams has written some arti- 

 cles on the rotation of the surface of Jupiter 

 in which he finds zones of constant cur- 

 rents. These speculations are not sound, 

 for the reason that in the same latitude we 

 find different rotation periods for the same 

 instant of time, and, as I have said before, 



