126 
Dr. Robinson, therefore, and his friend, had but little time 
for observation. He was, however, much interested by the 
vicinity of the craters named Hansteen and Mairan, in the 
map of Beer and Medler, where, besides the crowd of hills 
described by them, these are an infinity of others not visible 
even in the three-feet, but looking in this with 560 like grains 
ofsand. Are these fragments ejected from the crater? If 
so, and if they oceur round others, it would explain what had 
always presented to him a great difficulty. The lunar craters 
differ widely from those of earth; and most in this, that their 
depression below the general surface is enormously greater 
than the elevation of their walls above it, while the area of 
the hollow is far greater than that of the latter. What, then, 
became of the materials which had once filled it? He had 
formerly supposed that they were in a fluid or gaseous state 
when ejected; but the fact just mentioned seems to give the 
true solution, and appears to account for them when combined 
with the consideration of the feeble gravity on the moon, which 
would permit the exploded fragments to be scattered over a 
far larger space than with us. Another beautiful object was 
the river-like valley that runs northward from the crater He- 
rodotus: its raised banks, and their irregularities, were easily 
seen; the internal and external shadows could have been satis- 
factorily measured had a micrometer been applied. As it was, 
the much greater breadth of the former showed at a glance 
that this strange channel was sunk deep below the lunar sur- 
face. ‘Taking-as a standard the measures given there by Beer 
and Meedler, he had no doubt that they then saw without 
difficulty spaces of eighty or ninety yards. It is difficult to 
say @ priori what should be the minimum visible at the moon 
in such a telescope. If we assume, as one extreme, the state- 
ment of Amici, that the non-coincidence of two black lines on 
paper can be seen at twenty-eight feet, when it amounts to 
one-twelfth of an inch, or subtends 51”, then 311 feet should 
be visible at the moon with 1000. On the other hand, Jurin 
