In treatises upon physics and mechanics inertia is defined as follows: No particle of matter in a state of rest possesses within itself the power of putting itself in motion; or, if it be moving, of bringing itself to a state of rest.

As an example of this principle, we may recall here the trick performed by certain jugglers, and that consists in making a coin roll over the top of a Japanese paper parasol. The parasol is revolved very rapidly, and, to the eyes of the spectator, the coin seems to remain immovable. It is, in reality, the parasol that revolves under the coin.