A GIANT PLANET. 289 



So that these orbs may fairly be regarded as bearing the same relation 

 to their primary that Jupiter himself bears to his primary the sun. 

 It will be seen presently that this consideration is an important one. 



But the great interest of the study of Jupiter resides in the fact 

 that, being the nearest of the outer family of planets, the aspect of his 

 globe supplies the best available means of determining the condition 

 of the giant orbs constituting that family. 



The first feature which strikes us in the telescopic aspect of the 

 planet is the presence of a series of belts, lying parallel to the planet's 

 equator. Usually the equatorial regions are occupied by a broad, bright 

 belt, of a creamy-white color, and bordered on the north and south by 

 copper-colored belts. Beyond these, again, lie alternate bright and 

 dark belts, the dark belts growing more and more bluish in hue as the 

 pole is approached while the poles themselves are usually of a some- 

 what decided blue color in telescopes adapted to display such features 

 to advantage. There are commonly two or three dark belts on each 

 hemisphere. 



Now, before inquiring into the peculiarities presented by these 

 belts, and into the remarkable changes which have been noted lately 

 in their general aspect, it may be well for us to consider briefly what 

 such belts seem to imply. That they are due to peculiarities in the 

 planet's atmosphere is admitted on all hands. And it has been usual 

 to compare them with the trade-wind zones and the great equatorial 

 calm zone on our earth. The bright belts, according to this view, are 

 regarded as zones where for the time clouds are prevalent, the dark 

 belts being regions where the comparatively dark hues of the planet's 

 surface are brougrht into view. And then it has been deemed sufficient 

 to point out that the parallelism of the zones is due to the extreme 

 rapidity of the planet's rotation. 



But, setting aside the fact that the trade-wind zones and the great 

 equatorial calm zone on our earth are, in reality, little better than me- 

 teorological myths, it must be regarded as a remarkable fact that, in 

 the case of a planet so far away from the sun as Jupiter is, there 

 should be a supply of clouds so abundant as to form belts discernible 

 from the earth. Jupiter is rather more than five times farther from 

 the sun than the earth is, and receives from him about one twenty- 

 seventh part of the light and heat which falls upon the earth (equal 

 surface for equal surface). Making every allowance for the possibility 

 pointed out by Prof. Tyndall, that some quality in Jupiter's atmos- 

 phere may prevent the solar heat from escaping, and so cause the cli- 

 mate of the planet to be not very different from the earth's, yet the di- 

 rect heat falling on the planet's oceans cannot be increased in this way 

 nay, it must be rather diminished. It chances, indeed, that the very 

 quality by which the earth's atmosphere retains the solar heat is un- 

 questionably possessed by Jupiter's atmosphere. When our air is full 

 of aqueous vapor (invisible to the eye), the escape of heat is prevented, 

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