tic velocities. So far as we can estimate
the temperature at the boundary of the earth's atmosphere, we may assume
that the average of the velocities of the oxygen molecules there found
is about a quarter of a mile per second. The velocities for nitrogen are
much the same, while the average speed of a molecule of hydrogen is
about one mile per second, being, in fact, by far the greatest molecular
velocity possessed by any gas.

[Illustration: PLATE IX.

PLATO.

(AFTER NASMYTH.)]

A stone thrown into the air soon regains the earth. A rifle bullet
fired vertically upwards will ascend higher and higher, until at length
its motion ceases, it begins to return, and falls to the ground. Let us
for the moment suppose that we had a rifle of infinite strength and
gunpowder of unlimited power. As we increase the charge we find that the
bullet will ascend higher and higher, and each time it will take a
longer period before it returns to the ground. The descent of the bullet
is due to the attraction of the earth. Gravitation must necessarily act
on the projectile throughout its career, and it gradually lessens the
velocity, overcomes the upward motion, and brings the bullet back. It
must be remembered that the efficiency of the attraction decreases when
the height is increased. Consequently when the body has a prodigiously
great initial velocity, in consequence of which it ascends to an
enormous height, its return is retarded by a twofold cause. In the first
place, the distance through which it has to be recalled is greatly
increased, and in the second place the efficiency of gravitation in
effecting its recall has decreased. The greater the velocity, the
feebler must be the capacity of gravitation for bringing back the body.
We can conceive the speed to be increased to that point at which the
gravitation, constantly declining as the body ascends, is never quite
able to neutralise the velocity, and hence we have the remarkable case
of a body projected away never to return.

It is possible to exhibit this reasoning in a numerical form, and to
show that a velocity of six or seven miles a second directed upwards
would suffice to convey a body entirely away from the gravitation of the
earth. This speed is far beyond the utmost limits of our artillery. It
is, indeed, at least a dozen times as swift as a cannon shot; and even
if we could produce it, the resistance of the air would present an
insuperable difficulty. Such reflections,