our hours. The dial of Ahaz may have had a vertical gnomon on the upper one of a series of steps, the time being determined by the shadow of the point of the gnomon on the graduations of that arc-shaped step which was designed for that season of the year at which the observation was made.
It might thus resemble the analemma, described by Vitruvius, which, by marking the length of the shadows of a fixed gnomon, showed the different altitudes of the sun at the different seasons of the year.
Grotius supposed the dial of Ahaz to be a concave hemisphere with a central globe whose shadow fell on the lines engraved on the concavity.
This would resemble the Greek scapha, a semicircular concave dial, or hemicyclium, ascribed by Vitruvius to Berosus the Chaldean, 340 B. C.; this was long in use in Rome, and many have been discovered.
It consisted of a semi-spherical horizontal basin with a style erected in such a manner that its extremity was exactly at the center of the sphere.