340 On the Physical Cause of the Glacial Epoch, 



and consequently, of all the marks of a glacial valley, this would 

 be the one most likely to be first perceived. Two such terminal 

 moraines, one of them a double one, appear to me to be traceable 

 upon the moon's surface. The first is situated near the termi- 

 nation of that remarkable streak which commences near the base 

 of Tycho, and passing under the south-eastern wall of Bullialdus, 

 into the ring of which it appears to cut, is gradually lost after 

 passing crater 216 (Lubinietzky). Exactly opposite the last 

 crater, and extending nearly across the streak in question, are 

 two ridges curved towards the north, and reminding the observer 

 of the concentric moraines of the Rhone Glacier. Beyond the 

 second and outermost ridge a talus slopes gradually down north- 

 wards to the general level of the lunar surface. These ridges 

 are visible for the whole period during which that portion of the 

 moon's surface is illuminated ; but it is only about the third day 

 after the first quarter and at the corresponding phase of the 

 waning moon, when the sun's rays, falling nearly horizontally, 

 throw the details of this part of the surface into strong relief, 

 that these appearances suggest the explanation now offered. 



The other ridge answering to a terminal moraine occurs at 

 the northern extremity of that magnificent valley which runs 

 past the eastern edge of Rheita. This ridge is nearly semicir- 

 cular, and is considerably elevated, both above the northern ter- 

 mination of the valley, and the general surface of the moon. It 

 may be seen about four days after new and full moon; but the 

 position of the observer with regard to the lights and shadows 

 renders its appearance in the rays of the rising sun by far the 

 most striking. 



With regard to the probability of former glacial or even 

 aqueous agency on the surface of the moon, difficulties of an 

 apparently very formidable character present themselves. There 

 is not only now no evidence whatever of the presence of water, 

 in any one of its three forms, at the lunar surface, but, on the 

 contrary, all seleniographic observations tend to prove its absence. 

 Nevertheless the idea of former aqueous agency in the moon is 

 by no means new ; it was entertained by Gruithuisen and others. 

 But if water at one time existed on the surface of the moon, 

 whither has it disappeared ? If we assume, in accordance with 

 the nebular hypothesis, that the portions of matter composing 

 respectively the earth and the moon once possessed an equally 

 elevated temperature, it almost necessarily follows that the moon, 

 owing to the comparative smallness of its mass, would cool much 

 more rapidly than the earth ; for whilst the volume of the moon 

 is only about 4*9 th, its surface is nearly yWth that of the earth. 



This cooling of the mass of the moon must, according to all 

 analogy, have been attended with contraction, which can scarcely 



