The earliest horologe or hour measurer of which history makes mention is that called the _Polos_, and the _Gnomon_. Herodotus (lib. II.) ascribes their invention to the Babylonians, but Phavorinus claims it for Anaximander, and Pliny for Anaximenes. The _Gnomon_, which was the more simple and probably the more ancient instrument, consisted simply of a staff or pillar fixed perpendicularly in a sunny place, the shadow of which was measured by feet upon the place where it fell,--the flight of time being computed thereby. In later times the word _Gnomon_ was the title of the sun-dial, and it is the name still in use for the style or finger which throws the shadow on the dial and thus indicates the hour. The _Polos_ or _Heliotropion_ was no doubt a superior instrument to the earliest _Gnomon_, but, from its being so seldom mentioned, we may suppose it not to have been so generally used. The _Polos_ consisted of a basin, in the middle of which the perpendicular staff or finger was erected, and marked by lines the twelve portions of the day. The _Dial_ was but another form of _Polos_; its name indicates a Roman origin,--namely, from _Dies_, a day, but there was a Greek sun-dial called _Sciathericum_, from _skia_, a shadow. The invention is said to have been derived by the Jews from the Babylonians, to whom, as we have seen, Herodotus ascribed it, and there is mention made in the xxxviii. of Isaiah of the dial of Ahaz,--a king who began to reign 741 B.C. The form of the Dial of Ahaz has not been ascertained; but there is reason to believe that the ancient Jews and the Brahmins were acquainted with the uses of the dial and applied it to astronomical purposes. Dials were, it is said, not known in Rome before 293 B.C., when one was set up by Papirius Cursor the Roman General, near the Temple of Quirinus. At Athens there is an octagonal temple of the Winds still standing, which shows on each side the lines of a vertical dial and the centres where the _Gnomons_ were placed. At one time the art of Dialling was most assiduously studied; its rudiments may be described as follows: