248 
GILBERT. 
import of these ratios is materially modified by considera¬ 
tions arising from the laws of gravitational attraction. 
Taking into account the relations of the moon’s mass and 
radius to the earth’s mass and radius, it is computed that 
downward attraction at the moon’s surface is only one-sixth 
as great as at the earth’s surface. Bodies of the same size 
and material weigh only one-sixth as much on the moon; 
a bomb projected with the same energy or initial velocity 
would fly six times as far, and a cliff of the same material 
may stand six times as tall; so a lunar crater, if produced 
in the same way, may be six times as broad or deep as a 
crater on the earth without exciting our wonder. Applying 
the factor 6 to the ratios just cited, we reduce them severally 
to 9:1 and 4:1. 
E _mi. . u, 
Ml---UL1I_I_._._i_—-1 
Fit?. 5.—Diagram showing the relative diameters of the ten largest terrestrial craters 
( E ) and the ten largest lunar (M). The lunar diameters are divided by six. The 
diameters are represented by the distances from the vertical line at the left to the 
short vertical lines. 
To these ratios, considered as obstacles to the acceptance 
of the volcanic theory of lunar craters, three comments are 
pertinent: 
(1) The individual terrestrial diameters on which the 
second ratio is based are closely grouped near their maxi¬ 
mum (Fig. 5), as though constrained by a limiting condi¬ 
tion ; the individual lunar diameters are widely scattered 
near their maximum, like the distances of aberrant shots 
from the bull’s-eye. Reasoning from these facts of distribu¬ 
tion, we should predict that the complete exploration of the 
earth will bring to light other craters about as large as those 
now known, but will discover none much larger; but we 
could not make a similar prediction as to the maximum 
crater on the opposite side of the moon. 
(2) The conditions affecting volcanic action in the earlier 
