A GIANT PLANET. 93 



are warmer than where the air is dry. Now in Jupiter's 

 atmosphere there is much water, for observers armed 

 with that wonderful instrument, the spectroscope, have 

 recognised 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. The spectro- 

 scopist knows that these bands are due to the aqueous 

 vapour in the air, because Janssen saw the very same 

 bands when he examined the spectrum of a powerful 

 light shining through tubes rilled with steam. So that 

 there is the vapour of water and that, too, in enor- 

 mous quantities in the atmosphere of Jupiter. But 

 though we thus recognise 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 vapour finds its 

 way thus into the planet's atmosphere, as to understand 

 how the great cloud-masses are formed. 



Aqueous vapour 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 

 vapour in the air. In the summer, for instance 

 though many imagine the contrary there is much 

 more of such vapour in the air than there is in winter, 

 the greater heat of the air enabling it to keep a greater 

 quantity of the vapour in the invisible form. In winter, 

 clouds are more common, and the air seems moister ; 

 yet, in reality, the quantity of aqueous vapour is 



