THE MOON’S FACE. 
269 
that greater refinement seems not to be required ; but the 
theory of incidence angle nevertheless offers an inviting 
field to the mechanist. From the position reached in con¬ 
nection with the present study it seems probable that the 
moonlets originally moving in orbits outside the track of 
the moon would (mostly) reach the inner face of the moon 
(toward the earth), and the moonlets originally moving 
inside the moon’s orbit would reach its outer face. Ap¬ 
proaching the moon in this way, or in any other systematic 
way, the moonlets would determine and regulate the rotation 
of the moon. The motion of each moonlet at the instant of 
collision may be conceived as resolved into two components, 
one normal to the moon’s surface and the other tangential. 
If the tangential component coincided in direction and 
velocity with the rotational motion of the moon’s surface^ 
the collision would not affect the moon’s rotation ; but if the 
tangential component had a velocity greater or less than the 
rotational motion, the moon’s rotation would be accelerated 
or retarded. The aggregate result of all collisions would be 
such a rotation of the moon that its surface speed would 
equal the average of the tangential components of the veloci¬ 
ties of moonlet impact. It is evident that if the tangential 
component of a moonlet’s motion coincided exactly with the 
motion of the moon’s surface, the impact phenomena would 
be the same as though the moonlet fell vertically on a motion¬ 
less surface; and the harmonious adjustment of moon rotation 
to the motions of a system of moonlets would reduce to a 
minimum the ellipticity of craters. 
In fine, the hypothesis of the Saturnian ring, by restrict¬ 
ing the colliding bodies to a single plane, by substituting a 
low initial velocity and thus rendering the moon’s attrac¬ 
tion the dominant influence, and by introducing a system 
of directions controlling, and therefore adjusted to, the 
moon’s rotation, relieves the meteoric theory of its most 
formidable difficulty. It also explains in a simple way the 
abundance of colliding bodies of a different order of magni¬ 
tude from ordinary meteorites and aerolites. The remainder 
