Time is, strictly speaking, of infinite duration; we are, therefore, obliged to choose some arbitrary unit by means of which a measurement of time may be effected. For short intervals, the diurnal rotation on its axis of the globe we inhabit; for longer intervals, the annual revolution of the Earth around the Sun, are the standards of measurement we employ; but any event which takes place at equal intervals of time may serve the purpose of a chronometrical register. Thus, the ages of certain trees may be ascertained by counting the number of concentric rings in the trunk, so many being formed annually; the ages of certain cattle, by the number of rings on the horns; the ages of horses may in like manner be ascertained by the successive disappearance of marks from their teeth; so also the pulsations of the heart, the flowing of a certain quantity of water from one vessel to another, the oscillations of a pendulum, may all be used as measurers of time: but, in practice, the length of a day is a natural interval of time, which the domestic habits of man force upon him; and accordingly we find that amongst all nations this unit of measurement is, under some form or other, the one adopted.