d, to throw far, a run
is taken and the body is jerked forward; the arm is also moved as
rapidly as possible on the shoulder as pivot. The fore-arm can be
moved still faster, and the wrist-joint gives yet another motion:
the art of throwing is to bring all these to bear at the same
instant, and then just as they have all attained their maximum
velocity to let the ball go. It starts off with the initial
velocity thus imparted, and is abandoned to gravity. If the vehicle
were able to continue its motion steadily, as a balloon does, the
ball when let go from it would appear to the occupant simply to
drop; and it would strike the ground at a spot vertically under the
moving vehicle, though by no means vertically below the place where
it started. The resistance of the air makes observations of this
kind inaccurate, except when performed inside a carriage so that
the air shares in the motion. Otherwise a person could toss and
catch a ball out of a train window just as well as if he were
stationary; though to a spectator outside he would seem to be using
great skill to throw the ball in the parabola adapted to bring it
back to his hand.

The same circumstance enhances the apparent difficulty of the
circus rider's jumping feats. All he has to do is to jump up and
down on the horse; the forward motion which carries him through
hoops belongs to him by virtue of the motion of the horse, without
effort on his part.

Thus, then, it happens that a stone dropped sixteen feet on the
earth appears to fall straight down, although its real path in
space is a very flat trajectory of nineteen miles base and sixteen
feet height; nineteen miles being the distance traversed by the
earth every second in the course of its annual journey round the
sun.

No wonder that it was thought that bodies must be left behind if
the earth was subject to such terrific speed as this. All that
Copernicus could suggest on this head was that perhaps the
atmosphere might help to carry things forward, and enable them to
keep pace with the earth.

There were thus several outstanding physical difficulties in the way of
the acceptance of the Copernican theory, besides the Biblical
difficulty.

It was quite natural that the idea of the earth's motion should be
repugnant, and take a long time to