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 
