260 
GILBERT. 
of the rim to the capacity of the hole. If the collision pro¬ 
duced no condensation of the lunar tract affected (and con¬ 
densation would be anticipated only on the hypothesis of 
cosmic dust), we should naturally expect to find in the rim 
the entire volume of matter displaced in the formation of 
the hollow plus the volume of the moonlet; but this relation 
does not appear to obtain. The impression derived from 
telescopic observation and the inspection of photographs is 
that the rims of some craters are commensurate with the 
hollows, while the rims of others are not, and this impres¬ 
sion is confirmed by computations based on the measure¬ 
ment of shadows. Ebert has compiled the available pub¬ 
lished data and computed the ratio of rim content to cavity 
content for ninety-two craters, ranging in diameter from 8 
to nearly 100 miles.* In twenty-eight instances he finds the 
rim content the greater; in the remaining sixty-four instances 
he finds it the smaller; and in about fifteen instances the 
rim volume is but a small fraction of the content of the 
cavity. He finds further that the rim is relatively small or 
the cavity relatively large in the case of the larger craters. 
Though the imperfection of the data gives a large probable 
error to the determinations, there can be no question of the 
general fact that in many instances the rims of large craters 
are quite inadequate to fill the cavities they surround. This 
, is an important fact, but it is not necessarily inimical to the 
impact theory. In the course of a series of laboratory ex¬ 
periments, in which craters were produced by throwing pro¬ 
jectiles of various plastic materials against targets of similar 
materials, it was occasionally found that the rim when pared 
away would not fill the hollow, and the cause of this result 
was discovered. When target and projectile were of uni¬ 
form consistency throughout, there was no defect of rim; 
but when the general mass of the target was softer than the 
portion at the surface, the uplift consequent on the pro¬ 
duction of the hollow was only partly localized about its 
* H. Ebert: Ueber die Ringgebirge des Mondes. Sitzungsberichten d. 
Physik.-med. Societal. Erlangen, p. 171. Munich, 1890. 
