ASTKOXOMT AND METEOEOLOGT. 329 



tains bad been computed, and amongst them occur all degrees of alti- 

 tude up to nearly twenty-three thousand feet a height exceeding, 

 by more than a thousand feet, that of Chimborazo in the Andes. It 

 is a remarkable circumstance that the range of lunar Apennines, as 

 they have been called, presents a long slope on one side, and precipices 

 on the other, as in the Himalaya Mountains. During the increase of 

 the moon, its mountains appear as small points or islands of light be- 

 yond the extreme edge of the enlightened part, those points being the 

 summits illuminated by the sunbeams before the intermediate plain ; 

 but gradually, as the light advances, they connect themselves with it, 

 and appear as prominences detached from the dark border. 



The moon, unlike the earth, has many isolated mountains, that is to 

 say, mountains not connected with a group or chain, the mountain 

 named Tycho, which has the appearance of a sugar-loaf, is an example 

 of this. The uniformity of aspect which the lunar mountains, for the 

 most part, present is a singular and striking feature. They are won- 

 derfully numerous, especially toward the southern portion of the disk, 

 occupying quite the larger part of the moon's surface, and are, as Sir 

 John Hcrschel remarks, almost universally of an exactly circular or 

 cup-shaped form, foreshortened, however, into ellipses towards the 

 limb. The larger of these elevations have, for the most part, flat plains 

 within, from which a small steep conical hill rises centrally. They 

 offer, indeed, the very type of the true volcanic character, as it may 

 be seen in the crater of Vesuvius, and in a map of the volcanic dis- 

 tricts of the Campi Phlegrasi or the Puy de Dome, but with the re- 

 markable peculiarity, that the bottom of the crater is in many instan- 

 ces very deeply depressed below the general surface of the moon, the 

 internal depth being often twice or three times the external height. It 

 has been computed that profound cavities, regarded as craters, occupy 

 two-fifths of the surface of the moon. One of the most remarkable of 

 these formations is fifty-five miles in diameter ; and, to give some idea 

 of its magnitude, the late Professor Nichol used to say that, could a 

 visitor approach it, he would see rising before him a wall of rock twelve 

 hundred feet high ; and on mounting this height, would look down a 

 declivity or slope of thirteen thousand feet, to a ledge or terrace, and 

 below this would see a lower deep of four thousand feet more ; a cav- 

 ity exceeding, therefore, the height of Mont Blanc, and large enough- 

 to hold that mountain besides Chimborazo and Teneriflfe. Again, the 

 lunar crater, called Saussure, is ten thousand feet in depth. These 

 astounding calculations are founded on the observation of the sun's 

 light falling on the edge, and illuminating the side of these gigantic 

 depths. The Dead Sea, the greatest known depression on the earth, 

 is thirteen hundred and forty feet below the level of the Mediterra- 

 nean. 



Stride or lines of light, which appear like ridges, radiate from many 

 of these enormous craters, and might be taken for lava-currents, stream- 

 Ing outwards as they do in all directions, like rays. The ridges that 

 stream from the mountain called Tycho seem to be formed of matter 

 that has greater power of reflecting light than the rock around it ; the 

 crater named Copernicus is equally distinguished by these rays. The 

 ridges, in some instances, cross like a wall both valleys and elevations, 

 and traverse the plains as well as the rocky slopes of the lunar moun- 



28* 



