Replicas and Restorations of Early Instruments by Richard A. Paselk

Richard Paselk, Curator

Reflecting Quadrant or Octant, c. 1800

The octant measures angles up to 90° (a quadrant) using a 45° arc (an octant) and reflecting optics that double the angle. The ivory scale is graduated to 20' and can be read to 1' using the vernier. It was invented c. 1730 to help solve the “longitude problem.” Because of its ease of use and greater precision the octant soon displaced earlier altitude measuring navigational instruments such as the quadrant, the cross-staff, the mariner's astrolabe and the backstaff. Expansion of the measured angle to 120° in 1757 resulted in the sextant, the iconic instrument of celestial navigation. However, due to their lower cost and ruggedness octants continued in use throughout the nineteenth century. Even on ships carrying a sextant, an octant was often present as well, being used for everyday observations, particularly in less than ideal weather, saving the sextant for precision observations in calm conditions.

This octant has a radius of 13 3/4 inchs (index arm bearing center to vernier edge), while the greatest dimension of the ebony frame is 15 1/8 inchs. The scale is marked (between 45°&50°) with the Spencer, Browning & Rust "SBR" logo, showing it was graduated between 1785 and 1840 when this firm was in business.*

I obtained this octant in the Summer of 2012, for use in demonstrations and courses on instrumentation/navigation. The instrument was dirty and was missing some of the original brass furniture (see views of the upper half of the octant obverse and reverse). I restored it, including a light cleaning and waxing with Renaissance wax, and fabricating a variety of missing parts, as described below. The photo above is of the restored octant. All bright brass parts/furniture and the "ivory" pencil are replacements fabricated by me. Restored parts were left bright to show as replacements. Additional photos, including some detail shots of the restored instrument are seen below

Restored Octant

Replacement/Restored Brass Furniture

The missing mirror adjustment wingnut was first turned from brass round-stock to match the outer curves on the "wings" of the remaining original nut. The center was then drilled out to the depth of the wings and the excess brass cut off from the two sides with a hacksaw, then finished by filing. The half-round socket in the base was then created by filing to match the tab on the adjustment arm.

The tapered washers were turned from round-stock on a lathe to match the originals on the site and horizon mirror adjusting mechanism. Original washer dimensions d(see figure legends below) determined with dial calipers and Starrett small hole gauge.

The replacement backsite was fabricated in sections (note side-view above as well). It was then silver soldered onto a post turned from brass round-stock on a lathe. A hole was drilled in the post and threaded with a tap to fit a screw I had in my stash.

Replacement pencil and screwdriver

Ebony octants often have a hole in the cross-piece/brace to hold a pencil to note oberved angles on the small ivory plate on the frame back (see reverse image above), or less commonly a screwdriver to adjust the mirrors (note adjusting screws on mirror mounts in obverse octant image above).

Mirrors

I also made replacements for all three mirrors since the original silvering had badly degraded on all of them (Note the mirrors prior to restoration, the original mirrors are stored with the instrument). All replacement mirrors were cut from a thin glass commercial mirror-tile. The half silvered mirrors were prepared by scraping the backing (including the copper and most of the silver) off the desired clear window area with a razor blade and then dipping in 1 M nitric acid to remove the remaining silver. The resulting mirrors are quite functional for my purposes.

* What appears to be a nearly identical instrument but with original parts and micrometer adjustment can be seen at the NMAH (Smithsonian) collection.

** All photos were kindly provided by Richard Dunn, Senior Curator and Head of Science and Technology at the National Maritime Museum, Greenwich England (NMM), They are listed by the catalog numbers for the associated octants (photos etc. of all of these octants may be seen on the NMM website).

© R. Paselk 2013, Last modified 28 December 2020