The basic form of this theodolite--with the telescope mounted on the open side of a
semicircle--was introduced by Jonathan Sisson in London in 1737. In 1758 Edmund Stone described Sisson's theodolite as "certainly the best, most complete, handsome, and well designed
Instrument possible." In this example, the vernier for the horizontal circle is moved by rack and pinion, and reads to 12 minutes. The front side of the vertical arc also reads by vernier to 12
minutes; the back side has a logarithmic scale. The signature refers either to Jonathan Sisson
(about 1690-1749), or to his son Jeremiah Sisson (1720-1783).
Ref: William Gardiner, Practical Surveying (London, 1737).
N. Bion, Construction and Use of Mathematical Instruments, English translation by
Edmund Stone (London, 1758).