This theodolite was probably made in London in the mid-18th century. Its basic
form--with the telescope mounted on the curved side of a semicircle--derives from the design
that Thomas Heath introduced in 1725. The horizontal circle and vertical arc are graduated every
degree and read by verniers to 10 minutes. The words "Diff: Hypo & Base" on the telescope
support and the scales labeled "Feet" and "Links" on the vertical arc are used to correlate angle
of elevation or depression with horizontal distances when surveying sloping ground. In addition
to the telescope, there is a pair of open sight vanes. A level vial is mounted above the telescope.
This theodolite belonged to Orange Warner Ellis, a surveyor who lived in Odelltown, a
village along the Richelieu River, just a few miles north of New York State. Odelltown was
settled by Joseph Odell, a Loyalist from Poughkeepsie who moved from the United States to
Lower Canada in 1788, in order to remain under British rule. Odelltown would later be seen as a
Britannic outpost in a Francophone region of Quebec.
Ref: J. A. Bennett, The Divided Circle (Oxford, 1987), pp. 86, 146-147.