LUNAR MOUNTAINS. 



in the middle so as to cut in two the enclosed plain. In some rare 

 cases the enclosed plain is uninterrupted by mountains, and it is 

 almost invariably depressed below the general level of the sur- 

 rounding land. A few instances are presented of the enclosed 

 plain being convex. 



The mountainous circle enclosing these vast areas is seldom a 

 single ridge. It consists more generally of several concentric 

 ridges, one of which, however, always dominates over the rest, 

 and exhibits an unequal summit, broken by stupendous peaks, 

 which here and there shoot up from it to vast heights. Occasionally 

 it is also interrupted by smaller mountains of the circular form. 



25. The most remarkable of the class of lunar mountains, called 

 ring mountains, is that called TYCHO. This object is distinguish- 

 able without a telescope on the lunar disk when full ; but, owing 

 to the multitude of other features which become apparent around 

 it in the phases, it can then be only distinguished by a perfect 

 knowledge of its position, and with a good telescope. The enclosed 

 area, which is very nearly circular, is 47 miles in diameter, and 

 the inside of the enclosing ridge has the steepness of a wall. Its 

 height above the level of the enclosed plain is 16000 feet, and 

 above that of the external regions 12000 feet. There is a cen- 

 tral mountain, having the height 4700 feet, besides a few lesser 

 hills within the enclosure. 



This region of the moon is represented in the engraving 

 at the head of this tract, copied and reduced from the chart 

 of MM. Beer and Madler. The volcanic character observed 

 in the mountain formations loses much of its analogy to like 

 formations on the earth's surface when higher magnifying powers 

 enable us to examine the forms of what appear to be craters, and 

 to compare their dimensions with even the most extensive terres- 

 trial craters. Numerous examples may be produced to illustrate 

 this. Tycho, which, viewed under a moderate magniiying power, 

 appears to possess in so eminent a degree the volcanic character, 

 is, as has been stated, a circular chain enclosing an area of 47 

 miles in diameter. Gassendi, another system of like form, and of 

 still more stupendous dimensions, as seen with high magnifying 

 powers, consists of two enormous circular chains of mountains, the 

 lesser, which lies to the north, measuring 16^ miles in diameter, 

 and the greater, lying to the south, enclosing an area 60 miles 

 in diameter. The area enclosed by the former is therefore 214, 

 and by the latter 2827 square miles. The height of the lesser 

 .chain is about 10000 feet, while that of the greater varies from 

 3500 to 5000 feet. The vast area thus enclosed by the greater 

 chain includes, at or near its centre, a principal central mountain, 

 having eight peaks and an height of 2000 feet, while scattered 



