The Spot-Dial (1687)

Clerke (1626-1697?) gave private lessons in the mathematical arts and produced custom-made sundials. In 1682 he published Oughtredus explicatus, an edition of William Oughtred’s famous 1647 algebra textbook Clavis Mathematica. Clerke added a section on dialling, in which he described his invention of the spot-dial. Some years later, he was prevailed upon to write this brief English treatment of the dial. “The Use of this Dial is to see the Hour by the Rays of the Sun passing through a Hole, and terminated in a Glass set in a Frame within the House against the Window, the Ground of it is the same with that of the Sun’s Light passing through a Hole in the Window, and intercepted by a String, and of the Ceiling-Dial, where the Reflecting-Glass is supposed to be a Point in the Style.”