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THE POPULAR SCIENCE MONTHLY.— SUPPLEMENT. 



Until the moon is nearly six days old this crater 

 is in darkness. About that time the sun's rays 

 begin to fall on the region round the crater, caus- 

 ing the interior of the crater to appear as a mi- 

 nute dark spot. A day or so later the darkness 

 of the spot is notably reduced, because the light 

 begins to fall upon the floor of the crater. When 

 the moon is seven and a half days old a grayish 

 spot with diffused edge is seen. When the moon 

 is about nine days old the gray spot can hardly 

 be detected, and a few hours later no trace of the 

 crater can be perceived. 



But it may seem to the reader who chances 

 not to be familiar with astronomical relations 

 that if the spot is visible for three days about 

 the time of first quarter, and for three days also 

 about the time of third quarter, that allows the 

 lunarian some six days per lunar month in which 

 the new crater might be seen. This should be 

 ample time to study the moon so closely over the 

 region near Hyginus that the new crater could 

 not possibly escape detection — at least not when 

 such observations were continued for many years. 



The opportunities, however, are not quite so 

 favorable as might thus be supposed. The moon, 

 at her first quarter, follows the sun by about six 

 hours. As the sun is highest at noon, when due 

 south, so the moon at her first quarter is highest, 

 and therefore most suitably placed for observa- 

 tion, at six o'clock in the evening (when she is 

 due south). If the sun has not set at this hour — 

 in other words, during the six summer months — 

 the moon will not be so well seen as if the sun 

 were below the horizon ; for a veil of illuminated 

 air will be spread over her face. Again, for 

 about two months before and two months after 

 the autumnal equinox, the moon at her first 

 quarter attains but a low elevation when due 

 south, precisely as the sun is low at noon for two 

 months before and after mid-winter. According- 

 ly, it is only from November to May that the 

 moon can be well studied near her first quarter, 

 supposing the weather clear at or about six in 

 the evening, when she is at her highest above the 

 horizon. As there are thus only some eighteen 

 or twenty days in the year on which, for about 

 an hour on the average, the moon can thus be 

 favorably studied, and the like number from July 

 to February when the moon at her third quarter 

 can be so studied, 1 we see that the opportunities 

 for detecting any particular small crater are not 



1 The study of the moon at her first and third quar- 

 ters need not absolutely cease even at midsummer, 

 despite the veiling of her face when due south by sun- 

 lit air. 



so numerous as at first sight they might be sup- 

 posed to be. 



Nevertheless, even when these considerations 

 are fully taken into account, and also the circum- 

 stance that when the moon is favorably situated 

 and illuminated for the observation of Hyginus 

 and its neighborhood many other objects of inter- 

 est would be apt to attract attention, it cannot 

 but be regarded as a remarkable circumstance 

 that a crater so easily seen as Klein's should so 

 long have escaped detection if it really existed. 

 The improbability of this happening is so great 

 as to render extremely probable the inference 

 (which a recent writer has somewhat rashly de- 

 scribed as "absolutely certain") that, previous 

 to 1876, there did not exist on this portion of the 

 lunar surface a deep black crater, three miles in 

 diameter. " If, therefore," the same writer con- 

 tinues, " the existence of Dr. Klein's new crater 

 be confirmed, it will form the strongest possible 

 evidence of a real change on the surface of the 

 moon, a change, moreover, of a volcanic nature." 



Unquestionably if the crater did not exist or 

 was not visible before 18*76, and can now be seen 

 (as certainly is the case), a change must have 

 taken place on the surface of the moon. It 

 would remain very doubtful, however, whether 

 the change was of a volcanic nature. I men- 

 tioned some time ago that the case of Linne af- 

 fords, in two noteworthy respects, more satisfac- 

 tory evidence of volcanic change than the case of 

 the supposed new crater. One reason I have al- 

 ready mentioned, the fact, namely, that in Linn6's 

 case we have positive evidence, whereas in the 

 case of the new crater we only have negative evi- 

 dence. The other reason is this : In Linn e's case 

 the change supposed to have taken place can 

 hardly be ascribed to any cause except some 

 form of volcanic energy ; in the case of the sup- 

 posed new crater, volcanic action is not at all 

 necessarily indicated. That a lofty wall circling 

 a circular region six or seven miles in diameter 

 should be thrown down (and so completely as to 

 leave only a ring of fragments separately undis- 

 cernible) by the effects of mere expansion and 

 contraction due to the sun's action, may well be 

 regarded as altogether improbable. But there is 

 nothing improbable in the breaking up of a floor 

 covering a cone-shaped opening some two miles 

 or so in interior diameter. If, in some exceed- 

 ingly remote era of the moon's volcanic history, 

 the cone-shaped cavity had been filled with lava 

 through the funnel-shaped opening at the bottom, 

 the lower portion of the lava remaining liquid 

 longer than the upper would eventually flow out 



