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 Msedler, 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 

 of sand. Are these fragments ejected from the crater ? If 

 so, and if they occur 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 tliere by Beer 

 and Msedler, he had no doubt that they then saw without 

 difficulty spaces of eighty or ninety yards. It is difficult to 

 say d 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 



