## Fundamental Ideas Concerning Motion

With much greater exactness and regularity than man alone could ever do the process continued quite naturally and without interruption. One could refer to the motion of the pendulum, and of the balance wheel of a watch. Just as the primary unit of length was derived from a great natural dimension, viz., the circumference of the earth, so the standard of time is that period which our earth requires for a single rotation around its axis, or more exactly, it is the mean time elapsing between successive transits of the sun across the meridian at any one place. This period is divided into 24 hours, every hour is divided into 60 minutes, and every minute into 60 seconds. Pendulums are generally designed so that their period of oscillation is exactly one second, and it so happens that such pendulums are about one metre long. The units of time in different countries were much less different than the units of length.