Hour-Glass, a kind of chro-nometer, employed by navigators, as well as by some • artisans and mechanics, to measure the passing of time, by means of the descent or running of sand, out of one glass into another.

The best hour-glasses are those which are filled with egg-shells, well dried in an oven, finely pulverized, and sifted; as they shew the passing of time with greater exactness than common sand.