x THE STARRY HEAVENS 379 



are of such essential service to navigation. 

 They carry our vessels in and out of port, and, 

 indeed, but for them many of our ports would 

 themselves cease to exist, being silted up by 

 the rivers running into them. The Moon is 

 also of invaluable service to sailors by enabling 

 them to determine where they are, and guid- 

 ing them over the pathless waters. 



The geography of the Moon, so far as con- 

 cerns the side turned towards us, has been 

 carefully mapped and studied, and may almost 

 be said to be as well known as that of our 

 own earth. The scenery is in a high degree 

 weird and rugged ; it is a great wilderness of 

 extinct volcanoes, and, seen with even a very 

 moderate telescope, is a most beautiful object. 

 The mountains are of great size. Our loftiest 

 mountain, Mount Everest, is generally stated 

 as about 29,000 feet in height. The moun- 

 tains of the Moon reach an altitude of over 

 42,000, but this reckons to the lowest depres- 

 sion, and it must be remembered that we 

 reckon the height of mountains to the sea 

 level only. Several of the craters on the 

 Moon have a diameter of 40 or 50 one of 



