816 TUAXSACTIOXS OF THE AmEUICAK INSTITUTE. 



vers, as well as the photographs of Rutherford, Draper, De La E,ne 

 and "VVliipple. 



Upon a model of this size the greatest height of the lunar moun- 

 tains, from the bottom of the basin to the apex of the rampart, would 

 be represented by the tliickness of an ordinary sewing needle, the 

 one-third of which would represent the maximum altitude of the 

 mountains above the common level of the moon's surface, and as this 

 would furnish such slight elevations that they would resemble mere 

 scratches, I was forced to increase the altitude six times ; but as we 

 are compelled to resort to the same means, and for the same reason 

 in modeling the mountains of the earth, I merely notice it as one of 

 the necessities which present themselves in constructing a model 

 small enough to be made available for popular purposes. 



Ring Mountains ani> Ray Systems. 



That the ring mountains and the system of rays which so fre- 

 quently proceed from them, are but the co-results of a common cause 

 it would be superfluous to argue, as it never has been disputed ; and 

 they may be considered the grand feature of lunar topogra])hy. 

 The basin called Tycho exhibits the most extensive example of this 

 character upon tlie hemisphere of the moon which is turned toward 

 us, though that of the basin called Copernicus has evidently been 

 upon a scale of equal magnitude; but as it belongs to an epoch much 

 more ancient, causes have been at work upon its structure producing 

 modifications which have denuded and smootlied its original aspect, 

 and consequently obliterated its ancient forms in *j)roportion as they 

 were less strongly marked. 



A perpendicular section cut through the center of the basin 

 Copernicus would be represented by this diagram. It is fifty-six 

 miles in diameter ; its circular rampart has an altitude of nearly two 

 miles above the floor of the basin, while from the center of the 

 inclosure a rugged mountain rises to the height of 2,400 feet. Tlie 

 basin of Tycho is two miles narrower tlian this, but the ranqiart wall 

 is one-third higher and correspondingly more rugged, broken and 

 picturesque, with a center mountain diftering in like relations from 

 that of its more ancient relative. 



The full moon photographs beautifully illustrate the system of 

 bright streaks which radiate from Tycho, several of which may be 

 traced from 600 to 1,000 miles, and one to a still greater distance 

 from tlie center. 



