THE MOON’S FACE. 
273 
duce fusion are somewhat rare. During the earlier discus¬ 
sion of the doctrine of the conservation of energy Hagenbach 
fired a leaden bullet against an iron target,* melting a por¬ 
tion of the bullet. By a computation involving the velocity 
of impact, the weights of the melted and unmelted portions 
of the bullet, and the physical constants of the materials, he 
was able to account for more than 90 per cent, of the energy, 
and the theoretic relations between molar motion and heat 
were thus substantially verified. In ordinary rifle practice 
in a shooting gallery the small leaden bullets fired against 
the iron targets are both fused and rendered incandescent 
by the shock. In the course of my own experiments a small 
amount of fusion was produced by firing a ball of Wood’s 
alloy against a target of the same material. 
Central hills have not been produced in the laboratory by 
the impact of rigid materials, it being found impracticable 
to conduct operations on a suffi¬ 
cient scale, but they are readily 
formed with semi-liquid sub¬ 
stances. If a drop of water be 
made to fall on a still surface 
of water the outward-moving 
annular wave at one instant encloses a crater; at the 
next instant a mound rises in the center of the crater. If 
for the water there be substituted a thin mud, the relations 
may readily be adjusted so that the viscosity of the material 
will arrest the motion in either phase. If the drop fall from 
a certain height it produces a' cup-like cavity with a smooth 
rim; if it fall from a somewhat greater height it produces a 
larger cup with smooth rim and with a smooth, dome-like 
hill in the center (Fig. 13). Though this experiment does not 
yield forms closely resembling those of the moon, it serves 
to illustrate the process of gravitational recoil in the forma¬ 
tion of a central hill. The peculiar conditions ascribed to 
the lunar phenomena, and especially the fact of local soften¬ 
ing and fusion, seem adequate to account for the observed 
Fig. 13.—Central hill formed experi¬ 
mentally by gravitational recoil. 
*Pogg. Ann., vol. 140, 1870, p. 486. 
36—Bull. Phil. Soc., Wash., Vol. 12. 
