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was possible until some change had been proved to have taken 
place. 
Within the last few years some important instances of pro- 
bable lunar change have been made known. Thus the crater 
Liune described by Lohrmann, Madler, and Schmidt as six or 
seven miles in diameter and very deep, has now quite disappeared, 
leaving only a white spot surrounding a small crater of scarcely 
one-tenth the former area. Other instances have also been made 
known of the disappearance of craters which had been described 
by the earlier selenographers, and of the appearance of other 
craters in places where no crater had been seen before. In none of 
these cases is the evidence perfectly decisive ; there is in all cases 
room left open for doubt, and consequently they have been 
waived aside by astronomers in general. 
Several years ago it was pointed out that to obtain a decisive 
instance of a real physical change of this nature, it would be 
essential for a crater of some size to make its appearance in or 
to disappear from one of some three or four regions in the 
moon which had been thoroughly well studied. There are 
certain small portions of the lunar surface which have been 
assiduously studied and drawn by many selenographers, includ- 
ing several well-known living astronomers. It was then pointed 
out that the region around Hyginus, Triesnecher, and Ukert 
had been so thoroughly well studied that were a crater of three 
or four miles in diameter to appear or disappear in this portion 
of the moon, the fact could be established with certainty. It 
was also pointed out that these regions formed only one-thou- 
sandth part of the visible surface, so that though there might 
be numerous changes occurring on the lunar surface, it would 
be unlikely that one should occur in this comparatively small 
area. 
On the evening of May 19, 1877, a German astronomer, Dr. 
Hermann J. Klein, was examining the portion of the lunar 
surface around the well-known crater, Hyginus, using a first- 
class telescope of 5J inches aperture. The moon had not quite 
reached its first quarter, so that the sun had only just risen 
above the horizon of this portion of the moon, and its rays fell 
very obliquely on the surface, throwing every inequality into broad 
relief from the long shadows that they cast. For nearly twelve 
years Dr. Klein had been in the habit of observing this region, 
so that it was perfectly familiar to him, and he was well 
acquainted with its appearance. To his great surprise he 
caught sight of a large black crater filled with shadow, lying 
to the north of Hyginus, and near the border of the level plain. 
He felt certain that this crater had not existed on any previous 
occasion when he had examined this region, for it seemed to be 
so conspicuous that he was positive he could not have over- 
