47 6 Proceedings of Royal Society of Edinburgh. [sess. 
smallest size that can be seen np to at least 140 miles. Some of 
the crater floors are above the general level, such as Gassendi close 
to Mare Humorum, and some more nearly at the same level, when 
they are situated in or near the maria. The general characteristic, 
however, of those that are not near the maria is to have their floors 
much depressed, even to the extent of thousands of feet in some 
cases. The forms of these craters can be fairly well imitated by 
firing bullets into a mass of lead. The cavity thus formed has 
always a raised burr round it, is much larger in diameter than the 
bullet, and is generally fairly round even when the bullet strikes 
obliquely, if not so obliquely as to glance off altogether. There is 
always a small cone left in the cavity, and the surface of the whole 
cavity can be seen to glow red-hot immediately after the shot is 
fired. In the case here illustrated the bullets were elongated 
leaden ones '22 inch in diameter, and the cavities were *44 inch 
diameter. The three shots down the middle were fired perpen- 
dicular and the others obliquely, but not at measured angles ; 
however, the mark on the right of the centre was roughly estimated 
to be at about 45°. 
