THE MOON’S FACE. 
271 
ino’ in the crater, and another resulted in modifications of 
the moon’s motions, changing its orbit, its orbital velocity, 
its axis, and its rotational velocity. The energy converted 
into heat might be regarded as the remainder after deduct¬ 
ing all other effects, and the resulting temperatures would 
be further conditioned by the distribution of heat in the 
colliding masses. 
Since the area of the moon’s surface directly struck by 
the moonlet is a function of the square of the diameter of 
the moonlet, while the energy applied to that area, being 
measured by the mass of the moonlet, is a function of the 
cube of its diameter, more energy would be applied to a 
unit of space in the case of large moonlets than in the case 
of small, and the temperatures caused by large moonlets 
would therefore be greater. To this relation I ascribe the 
restriction of inner plains, indicative of fusion, to the larger 
craters, and the same explanation applies less directly to 
the limited distribution of central hills. 
In the production of small craters by small moonlets I 
conceive that the bodies in collision either were crushed or 
were subjected to plastic flow, and in either case were molded 
into cups in a manner readily illustrated by laboratory ex¬ 
periments with plastic materials. The material displaced 
in the formation of the cup was built into a rim, partly by 
overflow at the edges of the cup, but chiefly by outward 
mass movement in all directions, resulting in the uplifting 
of the surrounding plain into a gentle conical slope. This 
outward and upward movement was accentuated, possibly 
through the agency of heat, about the immediate edge of 
the cup, occasioning the special elevation called the wreath. 
The cups thus formed, having dimensions commensurate 
with the strength of the lunar material, were stable and 
permanent. The impact of a larger moonlet produced a larger 
cup, and at the same time fused a portion of the material 
and softened other portions. The walls of this cup were so 
lofty that they could not sustain their own weight, and they 
were further weakened by the effects of heating; consequently 
