290 THE POPULAR SCIENCE MONTHLY. 



as Tyndall has shown, and thus the nights are warmer than where tha 

 air is dry. Now, in Jupiter's atmosphere there is much water, for ob- 

 servers, armed with that wonderful instrument, the spectroscope, have 

 recognized the very same dark bands upon the spectrum of the planet 

 which appear in the solar spectrum when the sun is low down, and 

 therefore shining through the lower and denser atmospheric strata. 

 Ths spectroscopist knows that these bands are due to the aqueous 

 vapor in the air, because Janssen saw the very same bands when he- 

 examined the spectrum of a powerful light shining through tubes filled 

 with steam. So that there is the vapor of water and that, too, in 

 enormous quantities in the atmosphere of Jupiter. But though we 

 thus recognize the very quality necessary for an atmosphere which is 

 to retain the solar heat, our difficulty is not a whit lessened ; for it is 

 as difficult to understand how the invisible aqueous vapor finds its way 

 thus into the planet's atmosphere, as to understand how the great 

 cloud-masses are formed. 



Aqueous vapor in the atmosphere, whether its presence is rendered 

 sensible to the sight or not, implies the action of heat. Other things 

 being equal, the greater the heat the greater the quantity of watery 

 vapor in the air. In the summer, for instance though many imagine 

 the contrary there is much more of such vapor in the air than there 

 is in winter, the greater heat of the air enabling it to keep a greater 

 quantity of the vapor in the invisible form. In winter, clouds are 

 more common, and the air seems moister ; yet, in reality, the quantity 

 of aqueous, vapor is reduced. Now, it cannot but be regarded as a 

 remarkable circumstance that, though the sun supplies Jupiter with 

 only one twenty-seventh part of the heat which we receive, there should 

 yet be raised from the oceans of Jupiter such masses of clouds as 

 to form veritable zones ; and that, moreover, above these clouds there 

 should be so large a quantity of invisible aqueous vapor that the 

 spectroscopist can recognize the bands of this vapor in the planet's 

 spectrum. 



Even more perplexing is the circumstance that the cloud-masses 

 should form themselves into zones. We cannot get rid of this diffi- 

 culty by a mere reference to the planet's rapid rotation, unless we are 

 prepared to show how this rotation is to act in forcing the cloud-masses 

 to become true belts. The whole substance of Jupiter and his whole 

 atmosphere must take part in his rotation, and to suppose that aqueous 

 vapor raised from his oceans would be left behind in the upper air, like 

 the steam from a railway-engine, is to make a mistake resembling that 

 which caused Tycho Brahe to deny the rotation of the earth, because 

 bodies projected into the air are not left behind by the rotating earth. 

 Nor is it conceivable that belts which vary remarkably, from time to 

 time, in position and extent, should be formed by sun-raised clouds in 

 the Jovian atmosphere, if the planet's surface is divided into perma- 

 nent lands and seas. 



