i5 
1877 J Physical Changes upon the Moon's Surface . 
have been on hundreds of different occasions carefully scru- 
tinised to detedt any difference that might exist between 
them, without the most marked dissimilarity which now 
exists being detected. 
The most difficult portion of the question remains, how- 
ever, still untouched ; for even were it granted that in the 
case of Messier there exists an instance of real physical 
change, the process by which that change could have been 
brought about seems to defy explanation. In the case of 
Linne we have a plausible and easy method of accounting 
for the observed phenomenon, but in Messier what it seems 
must have occurred is a slow squeezing of an immense 
crater plain out of shape. The nature of the forces and 
processes at work upon the surface of the moon, which could 
compress a circular crater 9 miles in diameter into an ellip- 
tical formation some 12 miles by 7, is such with which 
there at present exists nothing analogous on the surface of 
the earth ; and until some satisfactory theory could be de- 
vised which would show by what means this change in 
Messier could have been brought about, in a manner con- 
sistent with other selenographical faCts, it is not surprising 
that a strong reluCtance should exist to admitting that a 
change had occurred in the form and dimensions of Messier, 
even when backed by the extremely strong evidence in its 
favour that has been adduced. 
It is possible, however, that this difficulty may be avoided, 
for careful examination of the crater plain Messier and its 
neighbourhood suggests that, instead of a bodily compression 
of the entire crater, the observed change may have occurred 
from the gradual sliding of the north and south walls into 
the interior, and in so doing pushing the entire western wall, 
outwards and westwards, down an incline existing there. If 
this, explanation is found to be consistent with the present 
condition of the formations, it at once removes all the diffi- 
culties, because the process it involves — namely, the gradual 
slipping of the wall of a formation into the interior — is one 
of which numerous traces exist upon the moon. There 
would be little difficulty on the part of selenographers in 
pointing to a hundred instances where a like circumstance 
had occurred,' — one within Gassendi being a notable case, 
and one where a massive wall has been pushed back nearly 
10 miles. As far as is at present known, this explanation 
is in accordance with the condition of the surface imme- 
diately around Messier, but further observations with pow- 
erful telescopes are indispensable. Unfortunately the region 
of the moon containing Messier is one which cannot often 
