CONDITION OF THE LARGER PLANETS. 191 



tion is very much greater, lags behind, while matter 

 rushing downwards is carried in advance, and thus 

 cloud zones are formed. 



A fifth argument is derived from certain considera- 

 tions depending on the behaviour of sun-raised cloud- 

 masses in our own air, both with regard to the progress 

 of the day, and with regard to the progress of the year. 

 We know that, speaking generally, the clouds change as 

 the day progresses, and that this is specially the case 

 in those regions of the earth where regular zones exist. 

 The sun, in tropical regions, rises in a clear sky and 

 quickly gathers clouds together ; these remain till the 

 afternoon, when they become dissipated (usually with 

 violent disturbance, electrical and otherwise), and the 

 sun sets in a clear sky. As seen from Venus or 

 Mercury the cloud-belt would extend across the middle 

 of the earth's disc, but would not reach to the edge, 

 either on the west or sun-rising side, or on the east or 

 sun-setting side. Nothing of the kind is observable in 

 the cloud-belts of Jupiter. Not only do they extend 

 right across (though becoming fainter near the edges 

 because seen through deeper atmosphere), but cloud- 

 masses have been known to remain, quite recognisable 

 in contour, during many Jovian days, and even for 

 forty or fifty of our own much longer days. So also with 

 regard to the year. In Jupiter's case, indeed, the 

 effect of annual changes in the arrangement of clouds 

 would not be recognisable, simply because the planet's 

 equator is nearly coincident with the plane of Jupiter's 

 orbit. But in Saturn's case the inclination of the 



