462 The Moon. 



shade produce effects the reason of which is not always appa- 

 rent, except to the careful observer, and which are perhaps not 

 always fully attended to, even by artists, who ought to be 

 especially scrupulous in this respect, but which show their 

 fullest development in the moon. There it is clearly seen how 

 small a relation the actual amount of elevation bears to the 

 extent of shadow which it occasions in the nearly horizontal 

 illumiuation of the beginning or close of the lunar day, and 

 how slight a resemblance there often is, under such circum- 

 stances, between the outline of the substance and the shadow. 

 Every little insulated hillock will betray itself in this position by 

 so disproportioned a shade, that Schroter considered that ele- 

 vations of not more than eighty feet might thus be clearly dis- 

 tinguished near the " terminator," or general boundary of light 

 and darkness, even at our distance of nearly a quarter of a 

 million of miles. The shadow of a rounded summit will be 

 projected out into a long spire, so sharpened at the point as to 

 deceive the unwary spectator into the impression of its falling 

 from a tapering pinnacle ; inconsiderable ridges near the ter- 

 minator will bring in great encroaching notches or bays of 

 darkness ; the rings of craters will increase strongly in appa- 

 rent breadth ; the long gentle slopes which usually incline up- 

 wards to the foot of the wall, like a broad " glacis/' in military 

 language, and pass unnoticed under a higher sun, being thrown 

 up from the surrounding level to swell the general mass, while 

 the interior cavity seems also to open wider as the lower slopes 

 and terraces of its precipitous sides disappear one after another 

 in the advancing shade. Nor are these more familiar appear- 

 ances all that the observer will have to account for. He must 

 seek in the same laws of illumination the explanation of less ordi- 

 nary effects ; he may find a tapering spire of shadow distorted 

 from its regular outline by the uneven ground which it tra- 

 verses, or squared off suddenly before "reaching its termination, 

 because it is traversed at right angles by a comparatively incon- 

 siderable ridge ; he may perceive that the shade cast by a 

 peaked but broad-shouldered summit loses its sharpness with 

 the advance of the lunar forenoon, and at length disappears, 

 from becoming entangled among its own inferior buttresses, 

 while these alone project a black outline of an entirely altered 

 character upon the plain at their feet. The same results of the 

 unchanging laws of illumination, it need not be said, occur 

 equally upon the earth ; but here we are little sensible of them. 

 Surrounded by them, and enveloped in them, we are not in a 

 position to comprehend their full proportions, or to judge in- 

 tuitively how the relief of our landscape would appear in a 

 bird's-eye view at the distance of a quarter of a million of 

 miles. .And besides this important difference, we can never 



