The magnetic compasses that Americans used to determine property boundaries were inexpensive and expeditious but affected by magnetic variation—the fact that magnetic north seldom coincides with true north, and the relation between the two changes over time. The vernier compass solved this problem.
This example dates from around 1785, and is probably the first American instrument of its kind; similar instruments were made in Ireland. The vernier (not easy to see) is on the rim, at north. An inscription reads "David Rittenhouse PHILADELPHIA".
David Rittenhouse (1732–1796) was a skilled clock and instrument maker, man of science, and master of the American mint.
Ref: D. J. Warner, "True North—And Why It Mattered in Eighteenth Century America," Proceedings of the American Philosophical Society 149 (2005): 372–385.
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