altitudes; but mass is always the same.

If projected through space, a certain mass
would move so as to produce momentum, which
would be equal at all places on the earth's surface,
or at any altitude.

Gravity has been called weight, and weight
gravity. The real difference is plain if gravity
is considered as the attraction of mass for mass.
Gravity is generally known and considered as a
force which seeks to draw things to the earth.
This is too narrow.

Gravity acts in all directions. Two balls suspended
from strings and hung in close proximity
to each other will mutually attract each other.
If one has double the mass it will have twice the
attractive power. If one is doubled and the other
tripled, the attraction would be increased six
times. But if the distance should be doubled the
attraction would be reduced to one-fourth; and
if the distance should be tripled then the pull
would be only one-ninth.

The foregoing is the substance of the law,
namely, that all bodies attract all other bodies
with a force directly in proportion to their mass,
and inversely as the square of their distance from
one another.