THE GREATEST PLANET. 189 



and a quarter as much as a globe of water the same size. 

 This is quite incompatible with the supposition that his 

 bulk is largely made up of materials at all resembling in 

 density the solid substances on the earth. The true 

 explanation must no doubt be that the mighty planet is, 

 on his exterior at all events, largely composed of great 

 oceans of clouds and vapours of density and thickness ; 

 that these vast clouds weigh but little in comparison with 

 their bulk ; and that thus the apparent size of Jupiter is 

 swollen to dimensions greatly exceeding those to which he 

 would retire if the materials now in vapour were con- 

 densed into more liquid or solid forms. 



When we ponder on the inflated size of Jupiter, and 

 when we apply to his globe the laws of heat, of gases, and 

 of vapours, which we have learned from experiments, we 

 are conducted to a very important discovery. There is 

 only one method of accounting for this stupendous mass 

 of impenetrable clouds which so persistently envelop the 

 planet. Clouds cannot exist except there be heat to 

 produce the requisite vapours. If the heat be absent, the 

 clouds will resume the liquid state. We are therefore 

 forced to the conclusion that the vast mass of clouds 

 enveloping the great planet can only be sustained by the 

 supposition that there must be some great source of heat 

 available on Jupiter. It is natural to inquire whence this 

 heat can be derived. Let us first compare or contrast the 

 condition of Jupiter with that of our own earth. 



We have no doubt a liberal supply of clouds supported 

 in our atmosphere. But our clouds are neither so copious 

 nor so continuously impenetrable as those on Jupiter. An 

 astronomer on the latter planet, who gazed at us through 



