S2 The Outline of Science 



from the sun that the vaporisation of its oceans must necessarily 

 be due to its own internal heat. It is too hot for water to settle 

 on its surface. Like Jupiter, the great globe turns on its axis once 

 in ten hours a prodigious speed and must be a swirling, seeth- 

 ing mass of metallic vapours and gases. It is instructive to com- 

 pare Jupiter and Saturn in this respect with the sun. They are 

 smaller globes and have cooled down more than the central fire. 



Saturn is a beautiful object in the telescope because it has 

 ten moons (to include one which is disputed) and a wonderful 

 tern of "rings" round it. The so-called rings are a mighty 

 arm of meteorites pieces of iron and stone of all sorts and 

 sizes, which reflect the light of the sun to us. This ocean of mat- 

 ter is some miles deep, and stretches from a few thousand miles 

 from the surface of the planet to 172,000 miles out in space. Some 

 astronomers think that this is volcanic material which has been 

 shot out of the planet. Others regard it as stuff which would 

 have combined to form an eleventh moon but was prevented by 

 the nearness of Saturn itself. There is certainly no life in Saturn. 



Mars and Venus are therefore the only planets, besides the 

 earth, nn which we may look for life; and in the case of Venus, the 

 possibility is very faint. But what about the moons which attend 

 the planets f They range in size from the little ten-miles-widc 

 moons of Mars, to Titan, a moon of Saturn, and Ganymede, a 

 satellite of Jupiter, which are about 3,000 miles in diameter. 

 May there not be life on some of the larger of these moons? We 

 will take our own moon as a type of the class. 



A Dead World 



The moon is so very much nearer to us than any other heav- 

 enly body that we have a remarkable knowledge of it. In Fig. 

 14 you have a photograph, taken in one of our largest telescopes, 



