THE MOON. 495 



on our planet by the level of the sea), the absolute heights 

 are not to be compared strictly with each other, since the six 

 numerical results here given, properly express only the differ- 

 ences between the peaks and the immediately surrounding 

 plains or hollows.* 3 It is, however, very remarkable that 

 Galileo likewise assigned to the loftiest lunar mountains the 

 height of about 4 geographical miles (24,297 feet) " incirca 

 miglia quatro" and in accordance with the extent of his 

 hypsometric knowledge, considered them, higher than any of 

 the mountains on the Earth. 



An extremely remarkable and mysterious phenomenon which 

 the surface of our satellite presents, and which is only optically 

 connected with a reflection of light, and not hypsometrically 

 with a difference of elevation, consists in the narrow streaks 

 of light which disappear when the illuminating rays fall 

 obliquely; but in the full Moon, quite in opposition to the 

 Moon-spots, become most visible as systems of rays. They 

 are not mountain veins, cast no shadow, and run with equal 

 intensity of light from the plains to elevations of more than 

 12,780 feet. The most extensive of these ray-systems com- 

 mences from Tycho, where more than a hundred streaks of 

 light may be distinguished, mostly several miles broad. 

 Similar systems which surround the Aristarchus, Kepler, 

 Copernicus, and the Carpathians, are almost all in connec- 

 tion with each other. It is difficult to conjecture, by the aid 

 of induction and analogy, what special transformations of the 

 surface give rise to these luminous, ribbon -like rays, pro- 

 ceeding from certain annular mountains. 



The frequently mentioned type of circular configuration, 

 almost everywhere preponderating upon the Moon's disc, in 



43 For the six heights which exceed 19,182 feet, sec Beer 

 and Madler, pp. 99, 125, 234, 242, 330, and 331. 



