Measuring the Land and its Wealth

c. 1850 – c. 1950

Richard Paselk, Curator

Vernier Surveyors Compass

Description:

Length = 16.5”; needle = 6”; Compass box = 6 15/16” o.d. The rim graduated 0°–90°–0°–90°–0° to 30’. The scale reads counterclockwise, showing direction directly when sighting from S to N. The scales, levels etc. are best seen in the top view photograph of the instrument. The variation arc on the North arm extends 20° in either direction. As seen in the close-up photo of the vernier scale, vernier reads to one minute on inlaid silver scales. The vernier is adjusted with a tangent screw on the South arm. The screw has a range of approximately 20°, but accomplishes a total range of 10°W to 20°E by means of moving the screw reference bearing between two holes in the South arm. There are two level vials, and E–W vial on the South arm (3.5” long) and a N–S vial on the North arm (2 ¼” long). Neither vial is graduated. The needle arrest is broken (knob missing, arrest mechanism ring offset so that a new screw cannot be accessed without dissembling the compass box - note the short slot in the compass box visible in the side view).

The vertical sights each have two slits with top and bottom pin holes. The South sight has two larger sighting holes above and below the center on the lower sight and one centered on the upper sight. The North site has two holes on the lower sight and one on the upper. The North vertical site is 7” high (the South site is slightly higher—due to repair?). Each sight is aligned with two registration pins and held with a clamping screw. The South site has a pin-punch prick between the registration pins matching a similar punch prick on the south arm (covered by the site) to enable proper assembly. The South site has been broken near the base and repaired with a brazed joint. From its appearance the repair was made while the compass was still in use, and was done by the user or a local craftsman, not an instrument professional, as the repair is poorly finished. The repair is visible on the left sight in the side view above. A leather washer on the South site clamping screw served no apparent purpose and was removed for display. The clamping screw is slightly different in styled from the North screw and may be a replacement given the repair etc.

Case

The compass is in a fitted softwood (?) case with sliding top, o.d. = 17 ¾” x 8 1/5” x 3 1/5”. It is well made with hand-cut dovetails, as seen in the photos below but relatively simple and of inexpensive wood, perhaps in keeping with Roach’s early philosophy of providing instruments at lower prices than others. The case is well worn and has a dark patina. One of the keepers for the sites is missing, a wooden spacer is extant but split (stored in a plastic bag). A photograph of the compass in the case is shown below. The top slides in dovetail grooves on either side. The right edge toward the open edge has a 7+” splinter missing (the wood patina indicates a long term loss). The “finger holes” for opening the top appear to be an owner modification. The bottom is inset and held on with iron nails. There is a fragmentary label on the inside of the case lid.

Additional information on early surveyor's compasses, John Roach and a similar compass may be accessed by the links below:

Essay on the Surveyor's compass, Smithsonian NMAH

Essay on John Roach, Smithsonian NMAH

Picture of Surveyor and Chainman on Smithsonian NMAH.

© R. Paselk 2013, Last modified 31 December 2020