A NEW CRATER IN THE MOON. 



425 



again through the opening by which it had en- 

 tered the cavity. Such a process not unfrequent- 

 ly occurs in terrestrial volcanoes ; in fact, it 

 probably occurs in the majority of cases. The 

 upper portion of the lava remains, in such a case, 

 as a thick but not very strong or massive roof, 

 spanning over the space left vacant by the lava 

 which has retreated. The roof might break up 

 before long, under fresh volcanic action ; or it 

 might not only resist such action for countless 

 ages, but remain for millions of years without 

 yielding either to the various strains and press- 

 ures to which it would be exposed, as the sur- 

 rounding slopes and its own mass gradually con- 

 tracted with the cooling of the moon, or to the 

 effects of the alternate expansions and contrac- 

 tions resulting from the heat of the lunar day and 

 the cold of the lunar night. But we can readily 

 understand that, in the long-run, a floor of this 

 kind must give way, and when it gave way it 

 would break up altogether, the fragments falling 

 to the bottom of the cone-shaped cavity, and 

 some of them even falling through the funnel- 

 shaped entrance of the cavity into the open space 

 between the crust and the nucleus there. 



But here a difficulty will present itself to every 

 one who is at all familiar with the aspect of the 

 moon's surface in the telescope. If there had 

 always been a crater where one now certainly ex- 

 ists, and this crater had been filled, or nearly so, 

 with lava which had solidified and formed a floor, 

 some difference of tint would have been recogniz- 

 able where this lava-floor existed. In full moon- 

 light perhaps the place of the crater might not be 

 discernible ; but under a moderately high sun the 

 lava should have been distinguishable from the 

 surrounding surface. 



Now, here the photographs of the moon ob- 

 tained during the last thirty years or so may 

 serve us in good stead. It is true the crater is 

 rather a small one for photography to deal with 

 at present. Yet many objects not more than two 

 miles in diameter are shown in some of the fine 

 photographs taken by Rutherfurd, De la Rue, and 

 Draper. Traces of Klein's crater might fairly be 

 looked for in some of these views of the moon. 

 I say in some of them, not specifying those taken 

 nearly at the time of the moon's first and third 

 quarter; though, of course, it would be in these 

 that the crater as it exists now might be expect- 

 ed to appear, if it is not a new formation. If, 

 however, the crater-floor has recently broken up 

 in the way suggested above, the signs of the for- 

 mer existence of a spot of different lustre and 

 contour (of different texture, as it were) might be 



found as probably under high illumination as un- 

 der a low sun. 



Through the kindness of Dr. Louis Ruther- 

 furd, of New York, I possess a fine series of 

 twelve photographic views of the moon in vari- 

 ous phases, besides one magnificent copy of the 

 photograph taken on March 6, 1865. I turned 

 to the examination of these views, early in the 

 present year, with considerable interest. Pass- 

 ing over three representing the moon before sun- 

 light had reached Hyginus, I take next (though 

 not next in phase) the view which shows Hyginus 

 under the lowest illumination: viz., one taken 

 two days or so before third quarter, on Septem- 

 ber 16, 1870. In this view, which appears in my 

 treatise on the moon) I find a small dark spot al- 

 most exactly where Klein's crater should be ; but 

 I am inclined to doubt whether this dark spot is 

 not an accidental mark, due to some small dust- 

 flaw in the negative. Many such dust-marks are 

 found in the same photograph, most of them 

 being quite obviously distinguishable from the 

 dark round shadows representing the small cra- 

 ters. But Hyginus is too near the edge where 

 illumination terminates (technically called the 

 terminator) for an opinion to be formed on this 

 point — at least, from the positive in my posses- 

 sion. I hope soon to hear that Dr. Rutherfurd 

 has caused the negative to be examined. Next 

 comes the splendid view taken on March 6, 1865, 

 when the moon was about nine days five hours 

 old, supposing Rutherfurd to have photographed 

 her when nearly south at New York. Now, in 

 this view (of which, be it noticed, I have two 

 copies, one belonging to the series above-named, 

 the other greatly enlarged) I find the place of the 

 new crater occupied by a small spot of lighter 

 color than that of the Sea of Vapors. It is well 

 seen in both copies. Doubtless the negative, if 

 carefully examined, would show more details, 

 and possibly resolve very definitely the question 

 whether Klein's crater is a new one at all. But 

 certainly, whether a real change has taken place 

 or not, the crater, with its lava interior, existed 

 as far back as 1865. Since this covers the en- 

 tire period during which Klein states that he has 

 frequently observed this region without discover- 

 ing the crater, there can be no reason for believ- 

 ing that any change had occurred in the crater 

 before 1865. 



In a photograph taken on February 28, 1871 

 (forming the frontispiece of my " Moon "), when 

 the moon was nearly half a day older, the region 

 occupied by the crater is lighter than the sur- 

 rounding sea, but is less definitely indicated, be- 



