716 THE UNIVERSE. 



glasses enable us to see these extinct craters in such pro- 

 portions that none of their details escape us ; whilst, were 

 we on the moon, our telescopes, according to Humboldt, 

 would scarcely enable us to make out terrestrial volcanoes. 

 Seen from the earth, many lunar volcanoes appear very 

 much depressed, and the edges of their craters resemble so 

 many flattened rings, projecting very little above the plains. 

 Some regions are so riddled with them that their mouths 



259. Appearance of the Moon when Full. 



touch. Others surmount lofty summits, and their crenel- 

 lated ramparts surround enormous excavations, which pierce 

 deep into the mountains below the level of the plains. 



At the present day the volcanoes with which the moon 

 is riddled are quite extinct, and our satellite is really and 

 truly a dead star. This opinion, which is held by Beer 

 and Arago, has been developed in an ingenious manner by 



