x6 Physical Changes upon the Moon's Surface . [January, 
be observed at the proper time for this purpose ; so that only 
one or two days during the year are likely to be found when 
Messier is in a proper position to be observed, and the 
weather will permit a high power and a powerful telescope 
to be used to advantage, for these require very steady and 
favourable atmospheric conditions. 
It is not, however, in these two instances alone that 
selenographers have detected instances of changes in the 
lunar surface ; but these two instances are cases where there 
exist the very strongest evidence that real physical changes 
have occurred. There are various other remarkable cases 
which render it very probable that different small portions 
of the moon’s surface have undergone striking variations ; 
but though they rest on evidence that in other branches of 
astronomy have been usually accepted as sufficient to esta- 
blish the existence of such variations, they do not rest on 
evidence of so overpowering a weight as is now alone consi- 
dered worthy of credit on these selenographical questions. 
Thus towards the central portions of the moon, on the wide 
open plain called the Mare Nubium , according to Beer and 
Madler, when they drqw their map and whilst they were 
making their observations, there existed a fine, deep, glit- 
tering white crater, called by them Alpetragius d. It had a 
diameter of 5 miles, and close to it, on the south-west, was 
a smaller crater, about 1 mile in diameter. The large crater 
Alpetragius d has entirely disappeared, leaving in its place 
a slightly less bright, perfectly round, greyish white spot, 
with a diameter of 7*2 miles. The smaller crater, however, 
remains unaltered, and corresponds to Madler’s description. 
We have here, in fadt, an exactly similar case to the well- 
known instance of Linne. About 240 miles south of this 
last region, on the border of the grey Mare Nubium, is a 
fine though ruined walled plain, named Hesiodus, and con- 
sisting of a level plain nearly 20 miles in diameter, surrounded 
by the ruins of a mountain border. Beer and Madler describe 
and draw this formation as containing a perfectly level in- 
terior, grey in colour, and becoming slightly lighter towards 
the centre. Within it now, nearly in the centre, is a fine 
deep crater, 4 miles in diameter, and very distinct long after 
sunrise, so that it seems difficult to understand how it could 
have escaped Beer and Madler’s notice, had it then existed 
like it is now. Many other instances of a similar nature 
might be easily quoted. 
Selenographers have also been long acquainted with vari- 
ations of a different nature, occurring in lunar regions, and 
consisting in variation in colour or brightness. Beer and 
