s, in the first place, endeavour to think of a globe freely poised
in space, and completely isolated from the influence of every other body
in the universe. Let us imagine that this globe is set in motion by some
impulse which starts it forward on a rapid voyage through the realms of
space. When the impulse ceases the globe is in motion, and continues to
move onwards. But what will be the path which it pursues? We are so
accustomed to see a stone thrown into the air moving in a curved path,
that we might naturally think a body projected into free space will
also move in a curve. A little consideration will, however, show that
the cases are very different. In the realms of free space we find no
conception of upwards or downwards; all paths are alike; there is no
reason why the body should swerve to the right or to the left; and hence
we are led to surmise that in these circumstances a body, once started
and freed from all interference, would move in a straight line. It is
true that this statement is one which can never be submitted to the test
of direct experiment. Circumstanced as we are on the surface of the
earth, we have no means of isolating a body from external forces. The
resistance of the air, as well as friction in various other forms, no
less than the gravitation towards the earth itself, interfere with our
experiments. A stone thrown along a sheet of ice will be exposed to but
little resistance, and in this case we see that the stone will take a
straight course along the frozen surface. A stone similarly cast into
empty space would pursue a course absolutely rectilinear. This we
demonstrate, not by any attempts at an experiment which would
necessarily be futile, but by indirect reasoning. The truth of this
principle can never for a moment be doubted by one who has duly weighed
the arguments which have been produced in its behalf.

Admitting, then, the rectilinear path of the body, the next question
which arises relates to the velocity with which that movement is
performed. The stone gliding over the smooth ice on a frozen lake will,
as everyone has observed, travel a long distance before it comes to
rest. There is but little friction between the ice and the stone, but
still even on ice friction is not altogether absent; and as that
friction always tends to stop the motion, the stone will at length be
brought to rest. In a voyage through the solitudes of space, a body
experiences no friction; there is no tendenc