The variation arc on the south arm of this surveyor's compass extends 14 degrees to either side; the vernier is moved by hand, and reads to 6 minutes. The bottom plate is not solid, as was usually the case, but has six large segments cut out, presumably to save brass and/or reduce the weight of the instrument. The compass face reads clockwise--a feature that was probably introduced at the behest of the surveyor for whom the compass was made. Heisely also made compasses with the more common counterclockwise form. The level vials on the north and south arms, the needle, and one of the sight vanes seem to be later additions.
The "F. Heiseley Fredk: town" inscription is that of Frederick Heisely (1759-1843), a clock and compass maker of German descent who catered to the community of rich German American farmers in the mid-Atlantic region. He apprenticed with John G. Hoff, a clock maker born and trained in Germany. He served in the Revolutionary War, married Hoff's eldest daughter, and opened a shop in Frederick (then called Fredericktown), Md., in the mid-1780s. He moved to Harrisburg, Pa., in 1811.
Ref: Silvio A. Bedini, "Frederick Heisely: Clock and Compass Maker," Rittenhouse 3 (1989):120-124.
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