Nature, though she lias supplied us with, visible phenomena to measure the larger units of time, such as days, months, and years, has not furnished us with any means whereby we may measure the lesser units of hours, minutes and seconds; artificial contrivances must therefore be sought for. Hough approximations to the true time were at first obtained by setting up gnomons, or upright staves; which, in conjunction with a knowledge of the north point of heavens, would afford a tolerably correct indication of noon, or the moment of the Sun's passage over the meridian. An instrument constructed with a gnomon pointing towards the North Pole of the heavens, constitutes a sun-dial, and affords a still better mode of ascertaining the hour of the day. According to Herodotus, sun-dials were first introduced into Greece from Chaldæa; the hemisphere of Berosus, who lived 540 B.C., is the oldest recorded in history. The earliest attempt to form a strictly artificial time-keeper, is due to Ctesibius, of Alexandria, who invented Clepsydrœ, or water-clocks, which were contrivances for allowing a continuous stream of water to trickle through a small aperture in the pipe of a funnel, the time being measured by the quantity of fluid discharged.