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A week ago, I drew members' attention to a "Venerable sextant" that I had just finished restoring (g 13215, 9 June 2010). At the end of my account, I wondered how accurate was this 200-year-old vernier sextant, which had plainly lived hard towards the end of it active life.

Inside the lid of the case was a tattered Hughes and Son certificate with only the readings for 30 and 60 degrees present, both zero, which in the context of a vernier sextant could be interpreted to mean "less than the least graduation" in this instance, 30 seconds. The part of the certificate with the date had perished, but we can guestimate that it is from after about 1920, when the "Husun" trademark was first used. I could not resist spending a little time to re-calibrate it, with the following results:

Angle Error, seconds
15 +6
30 +2
45 -3
60 +10
75 +6
90 +3
105 +24
120 +25

These are every bit as good as for many a mid-twentieth century sextant and seem to show that vernier sextants, at least, do not wear out, though many arcs have been reduced to uselessness by polishing. The index arm bearing, being tapered, reacts to any wear by the journal (shaft) moving axially deeper into the bearing (socket), but in any case, a slow moving, lightly loaded bearing, used only intermittently is unlikely to wear significantly over several lifetimes of use.

I don't know whether Gilbert and Co possessed a Ramsden-type dividing engine, described by Jesse Ramsden for the Commissioners of Longitude in 1777. In return for a very substantial cash reward of 615 pounds, Ramsden also got to keep the machine, provided that he used it to divide instruments for other makers, for 3 shillings a quadrant and 6 shillings a sextant “…for so long as the said Commisioners shall think proper to permit the said engine to remain in his possession:…). At this period, the weekly wage of an unskilled worker was about 9 shillings. By 1800, there were perhaps half a dozen copies of the dividing engine in London and the Science Museum in London has three of them, while the Smithsonian Museum has Ramsden's original circular dividing engine.

Ramsden’s dividing engine was very important in the history of science as well as of navigation. My acquaintance with Gilberts’ old sextant re-kindled my interest in the art of dividing , so in the coming weeks I plan to scan my rather battered old photocopy of his “Description of an Engine for Dividing Mathematical Instruments” (perhaps with a commentary of my own) and post it on NavList as a reference source. Bibliobazaar have produced a facsimile version for those who can’t wait, but I don’t know whether the magnificent engravings of the instrument have been included in that edition.

Bill Morris
Pukenui
New Zealand

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