The mercury clock of the Libros del Saber

Abstract

Summary The Libros del Saber de Astronomia is a compilation of various Arabic astronomical works translated into Castilian in the second half of the thirteenth century, under the direction of King Alfonso X of Spain. A section describing a mercury clock has been suggested to be of particular significance in view of the likely invention of the mechanical clock around this period, so a new translation into modern technical English has been prepared. The clock is shown to consist essentially of an annulus of mercury-filled chambers separated by narrow partitions, each partition being perforated by a very small hole. This annulus is attached concentrically to a pulley wheel around which a cord is wrapped, and a fixed weight hung upon the cord. The torque raises the mercury on the opposite side, but as the resulting hydrostatic head then causes it to percolate through the holes connecting the chambers the wheel slowly rotates to maintain its equilibrium. The weight therefore gradually descends in a controlled ...