THE GREATEST PLANET. 191 



cloud. For an explanation of all terrestrial clouds, we 

 require no other heat than that which the sun dispenses 

 so liberally and unceasingly. 



In endeavouring to understand the clouds of Jupiter, 

 the first attempt which we naturally make is to explain 

 their origin in a manner similar to that of the clouds on 

 the earth. Jupiter revolves around the sun as this world 

 does, and like this world Jupiter is also illuminated by 

 the sun and warmed by him. Can it not therefore be 

 that the sunbeams on Jupiter generate the vast cloudy 

 covering just as they do here ? 



This is a point on which a simple calculation will throw 

 much light. The average distance of the great planet 

 from the sun is 5 times as great as the distance of the 

 earth. It can easily be shown that the intensity of the 

 heat received from any source decreases as the distance of 

 the source increases. The rate of decrease is indeed a 

 rapid one. If the distance be doubled, the supply of heat 

 is not alone halved, it is reduced to one-fourth of its 

 original intensity. In fact, we may express the law 

 generally in that form in which nature loves to work, 

 that the intensity of the heat received from a source 

 varies as the inverse square of the distance. It therefore 

 follows that to compare the sun heat received on Jupiter 

 with the sun heat received on the earth, we must square 

 the number 5 ; that is, multiply it by itself. The result 

 is very close to 27, and hence we learn the significant fact 

 that the intensity of sun heat on Jupiter is only one 

 twenty-seventh part of that which is received on the earth. 



We cannot believe that the sun's rays, enfeebled to a 

 twenty-seventh part of the intensity in v*hich we know 



