Surveyor's Vernier Compass
"David Rittenhouse PHILADELPHIA"
length 13.375 inches; needle 5.25 inches.
This unusual instrument may be the first vernier compass ever made. The vernier scale located at the northern edge of the face and reading to 6 minutes might be used to compensate for quirks in the magnetic needle (the fact that all needles did not point to the same direction was a common problem at the time), or to compensate for magnetic variation. The compass does not, however, have a variation arc, a feature that would soon become standard for vernier compasses. This compass must have been made after 1770, when David Rittenhouse moved to Philadelphia, and before his death in 1796; when, exactly, however, we do not know.
Since this compass has a broad baseplate and bar, it is substantially more rigid and reliable than those with just a narrow bar holding the vertical sights. Rittenhouse introduced this feature in the 1760s, and it was soon adopted by other American makers. The face of this compass reads counter clockwise (west is to the right of north). This feature was scarcely known in Britain. Rittenhouse introduced it to American practice, and it too was soon widely adopted. -