North American Sundial Society

Gilbert Clerke. THE SPOT-DIAL, Very useful to shew the Hour within the House.

"Together with Directions how to find a true Meridian, the Azymuth and Declination; and how to draw a Dial upon a Staff, upon the Cieling, upon any Pillar or Globe. Never Before Published. By Gilbert Clerke. London, Printed by J.M. for Walter Kettilby at the Bishop's-Head in St. Paul's Churchyard, 1687."    (30 pages, 1.3 MB)

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. “I did not think to have written any more of Dialling, having delivered my Mind sufficiently at the End of my Oughtredus Explicatus; but being desired by a Neighbour Gentleman to make a fuller Description of the Spot-Dial, which I mention there…, I have now undertaken it, according to that Gentleman’s Desire, having had much Experience (since that writing was extant) both of the best wayes of drawing the Dial, and of the best Materials, and so much as I believe, that little will be added by him, that comes after, to what I shall here say. 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.”