CONDITION OF THE LARGER PLANETS. 189 



level, then fifteen miles below that limit the pressure 

 would be equal to that of air at our sea-level, fifteen 

 miles lower one million times as great, and fifteen 

 miles lower yet, or still only sixty miles below the 

 visible limits of the cloud envelope of Jupiter, the 

 pressure would be one thousand million times as great 

 as at our sea-level. The density, if only the gases 

 composing that atmosphere could remain as perfect 

 gases, would be more than a million times greater than 

 the density of the heaviest known elements. Of course 

 there is no such pressure, no substance exists at that 

 density, sixty miles below the visible limits of Jupiter's 

 atmosphere, nor ten thousand miles lower yet. . No 

 gas could remain as such at ordinary temperatures 

 beneath a pressure which would make it as dense even 

 as water ; and if strata could and did exist in Jupiter 

 at the higher pressures and densities named, he would 

 weigh many thousand times as much as he actually 

 does. But we are again forced to the belief that, 

 unless his atmosphere is made of substances altogether 

 different from any with which we are acquainted, there 

 must be some power at work to prevent the compression 

 which would otherwise inevitably result from the 

 tremendous attractive energy of Jupiter's mass. That 

 power can be no other than the fierce heat with which 

 his whole frame, his atmosphere (and all but the exterior 

 strata outside the outermost cloud-layers) are instinct. 

 It appears to me that a fourth argument of very 

 great force can be derived from the cloud-belts in the 

 atmosphere of Jupiter and his brother giant, Saturn. 



