dth part of the gravity of
the earth. It follows that the weight of an object on the earth would be
reduced to the thousandth part if that object were transferred to the
planet. This would not be disclosed by an ordinary weighing scales,
where the weights are to be placed in one pan and the body to be weighed
in the other. Tested in this way, a body would, of course, weigh
precisely the same anywhere; for if the gravitation of the body is
altered, so is also in equal proportion the gravitation of the
counterpoising weights. But, weighed with a spring balance, the change
would be at once evident, and the effort with which a weight could be
raised would be reduced to one-thousandth part. A load of one thousand
pounds could be lifted from the surface of the planet by the same effort
which would lift one pound on the earth; the effects which this would
produce are very remarkable.

In our description of the moon it was mentioned (p. 103) that we can
calculate the velocity with which it would be necessary to discharge a
projectile so that it would never again fall back on the globe from
which it was expelled. We applied this reasoning to explain why the moon
has apparently altogether lost any atmosphere it might have once
possessed.

If we assume for the sake of illustration that the densities of all
planets are identical, then the law which expresses the critical
velocity for each planet can be readily stated. It is, in fact, simply
proportional to the diameter of the globe in question. Thus, for a minor
planet whose diameter was one-thousandth part of that of the earth, or
about eight miles, the critical velocity would be the thousandth part of
six miles a second--that is, about thirty feet per second. This is a low
velocity compared with ordinary standards. A child easily tosses a ball
up fifteen or sixteen feet high, yet to carry it up this height it must
be projected with a velocity of thirty feet per second. A child,
standing upon a planet eight miles in diameter, throws his ball
vertically upwards; up and up the ball will soar to an amazing
elevation. If the original velocity were less than thirty feet per
second, the ball would at length cease to move, would begin to turn, and
fall with a gradually accelerating pace, until at length it regained the
surface with a speed equal to that with which it had been projected. If
the original velocity had been as much as, or more than, thirty feet per
second, then the ball wo