14 Physical Changes upon the Moon’s Surface. [January, 
form, with a long diameter of about io to n miles and a 
short diameter of about 7J to 8 miles. The most rigid 
equality between the form and dimensions of these two 
formations, so strongly asserted to exist by Beer and Madler, 
then no longer existed in 1857, according to the observations 
of the Rev. T. W. Webb. 
This fadt was of very great selenographical importance, 
and should have attracted general attention, because what- 
ever uncertainty may be attached to the observations of 
Linne made by Beer and Madler, it cannot be seriously 
urged that Beer and Madler could have failed to recognise 
the dissimilarity between Messier and Messier A discovered 
by the Rev. T. W. Webb, when we consider that for ever 
three hundred times they most carefully examined the two 
formations for the express purpose of detecting any difference. 
Nor can that potent agent, the lunar librations, be invoked 
to account for this difference, for in this particular instance 
it is entirely without sensible influence. For Messier lying 
very close to the moon’s equator, and in longitude 47 0 W., 
no variation in the libration of the moon can sensibly affedt 
the meridianal apparent diameter of the formation, and it is 
in this last that the diminution has been observed. If it was 
established, therefore, that the two formations were no 
longer alike, but differed markedly in any respedt, the very 
strongest evidence of adtual physical change would be the 
consequence. 
Although therefore it is thus important that further ob- 
servations of these formations should be obtained, the matter 
attracted little attention until the period 1870 to 1875. 
During these years Messier and Messier A have been re- 
peatedly examined with the aid of most powerful telescopes, 
and the present dissimilarity between Messier and Messier A 
has been placed beyond the possibility of doubt. From 
some measures during the past year the long diameter of 
Messier appears to be 12*2 miles, and the short diameter — - 
which is nearly, though not quite meridianal — is 6*9 miles. 
The difference between the form and dimensions of the two 
formations is now obvious in the smallest astronomical 
telescope, for it is unmistakable with a power of 150 on a 
telescope with an aperture of a little over 2 inches. With 
the fine Frauenhofer equatorial of Beer and Madler, with an 
aperture of nearly 4 inches and a power of nearly 300, the 
difference now existing between the two formations would 
have instantly arrested the attention of any selenographer. 
It is inconceivable that the two formations could have pos- 
sessed their present form in Beer and Madler’s time, and 
