ANUARY 16, 1903.] 



SCIENCE. 



91 



three years have established the following 

 facts: 



1. The equatorial belt changes in both 

 size and position to a considerable extent, 

 but these changes are usually slow and 

 gradual. Occasionally, however, a marked 

 change may be observed in the features of 

 the belt in the course of a number of days. 



2. The fainter belts also are displaced 

 in latitude and in the amount of material 

 of which they are composed. The visibility 

 of the fainter markings and spots depends 

 in a considerable measure on the distance 

 of the planet from the earth. "When the 

 planet is at more than mean distance, the so- 

 called polar belts are very faint and some- 

 times invisible, even with a large telescope, 

 and are not brought into view until the 

 planet approaches toward opposition. This 

 fact I noticed particularly in the early 

 years of my observation on Jupiter, when 

 the observations were made as near the sun 

 as possible. 



3. The egg-shaped white spots, which ap- 

 pear in this form from perspective, as they 

 are probably nearly circular, are found 

 both north and south of the equator and 

 are very permanent in latitude. They are 

 usually from one to two seconds of arc in 

 diameter. These spots are not fixed with 

 regard to each other, even when they are 

 located in the same latitude. 



4. Aside from the white spots, there are 

 dark spots of similar size, sometimes on the 

 faint belts and sometimes entirely discon- 

 nected from the belt. The dark matter is 

 not as stable as the egg-shaped white spots, 

 and probably lies at the same level as the 

 equatorial belt. 



5. Near the equator are found white 

 spots, usually of a larger size and more ir- 

 regular in shape, which give rise to the 

 period of 9^ 50™. 



The mean density of the planet Jupiter 

 is 1.37 times that of water. The spheroidal 



figure of the planet indicates that the 

 density increases as we proceed from the 

 surface to the center. In the case of the 

 earth the density at the surface is about one 

 third the mean density, and assuming the 

 same rule for Jupiter, its surface density 

 would be 0.4 to 0.5 that of water. The 

 liquefaction of air and gases during recent 

 years enables us to imagine a medium which 

 would have the density corresponding to 

 that of the surface of the planet. The older 

 astronomers, of course, had no knowledge 

 of any substance between atmosphere and 

 liquid, and hence, in forming their theories 

 of the motions on the surface of the planet, 

 the theory was necessarily -atmospheric, but 

 there is now no excuse for maintaining an 

 atmospheric theory which wiU not account 

 for the phenomena observed. 



A probable theory of the constitution of 

 the planet should in some degree satisfy all 

 the phenomena observed. No one can draw 

 legitimate conclusions from casual observa- 

 tions. On the surface of Jupiter we find 

 the following objects: (1) The great red 

 spot, which is the most stable of aU objects 

 seen on the disk of the planet. During the 

 period that its size has been measured with 

 the micrometer one cannot say with cer- 

 tainty that there has been any change in 

 its size or shape from 1879 to 1902. It is 

 now conceded by astronomers that the ob- 

 ject is identical with the spot observed by 

 early astronomers. Such being the case, it 

 would seem to be absurd to say that any- 

 thing in the nature of a cloud should persist 

 in the same form for more than 200 years. 

 Its spheroidal shape in connection with its 

 stability would seem to show that it has 

 volume and mass. Its motion in latitude, 

 as we have already seen, is much less than 

 for the equatorial belt. The matter of 

 which it is composed is in a different condi- 

 tion to that of the belt. In 1880 I had the 

 good fortune to notice the transit of a satel- 



