13 
1 877.] Physical Changes upon the Moon's Surface. 
much the appearance of a bright comet with a long double 
tail. Schroter, of Lilienthal, discovered this objeCh, and 
drew it carefully, making the western formation of the two 
— or Messier itself — slightly the larger. He also suspeCted 
that its appearance was variable. In consequence of the 
observation of Schroter, Beer and Madler paid particular 
attention to these formations, examining them most care- 
fully on more than three hundred distinct occasions, 
between 1829 anc ^ I 837* They were thus enabled to declare 
with certainty that the two formations, Messier and 
Messier A, were exactly alike in every manner, not the 
slightest difference in any particulars being detectable. 
Both were circular crater plains, 9 miles in diameter, with 
7 0 bright greyish white walls, surrounding a yellowish grey 
interior only 3° bright. On the walls, which were of the 
same height, were several wall peaks, in both formations 
situated in the same position with regard to the formation. 
In faCt, as Beer and Madler draw particular attention to, 
their own observations are decisive proof that the two 
formations were exactly equal in every respeCt, and that in 
diameter, form, height of their walls above the surrounding 
surface, and depth of the interior beneath the crest of the 
walls, colour of the interior and of the walls, and the posi- 
tion of the surrounding wall peaks, Messier and its twin 
formation Messier A were completely alike. And it is im- 
possible that any difference in these respeCts would have 
escaped the attention of the greatest selenographer of our 
day, during a course of over three hundred observations in a 
space of nine years. 
Some years after this, Gruithuisen — a most zealous and 
keen-sighted, though fanciful, observer — detected a slight 
dissimilarity between the form of the two craters, but this 
faCt received little, if any, attention. In November, 1855, 
the Rev. T. W. Webb, one of the best living lunar observers, 
who was examining this region with a telescope of similar 
dimensions to that of Beer and Madler’s, observed that the 
eastern crater plain appeared the larger of the two. On 
again looking at the formations on March nth, 1856, he at 
once detected that not only was the western crater plain, or 
Messier the smaller of the two, but that it was elliptical, 
with its greatest diameter extending from east to west. 
This faCt was confirmed by subsequent observations and 
from drawings made by him in 1857, whilst Messier A, the 
eastern formation, appears to have remained unchanged, 
still being a circular crater plain, with. a diameter of 9 miles, 
the western crater plain, Messier itself,, had an elliptical 
