A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Do old sextants ever die?
From: George Huxtable
Date: 2010 Jun 16, 09:12 +0100
From: George Huxtable
Date: 2010 Jun 16, 09:12 +0100
I will look forward to seeing Bill Morris' comments about Ramsden's dividing engine. Sextant components were often assembled by the "maker" from a variety of sources, and it's quite possible that the scale had been divided by someone else for Gilbert. Perhaps by Ramsden, though a Ramsden job had a high reputation, and usually Ramsden marked such scales with his initials I R , although the instrument itself might be the product of another. Clearly, from Bill's measurements, the job was done precisely. Bill wrote, under "restoration", in www.sextantbook.com - "Engraved on the limb was the worn and barely visible inscription “King’s Patent. GILBERT & Co. Tower Hill. LONDON.” John Gilbert was a noted C18 instrument maker who at the time of his death in 1791 was working from 8 Postern Row, Tower Hill, in the shadow of the infamous Tower of London. The firm continued as Gilbert and Son and merged with another noted instrument maker in 1808, when the firms became Gilbert and Gilkerson. An identical sextant named “Gilbert and Son” is illustrated in Figure 48 of Peter Ifland’s book “Taking the Stars” and has been given an approximate date of 1790. I think mine is probably dated between 1791 and 1808, and I would be glad to hear from anyone who can date it more closely." I've been looking at what Eva Taylor had to say in her book "The mathematical practitioners of Hanoverian England" (1966). She dates Gilbert and Gilkerson as flourishing from 1792 to 1809, at the address given by Bill. John Gilbert, at a closely similar (probably the same) address, is dated by her to be flourishing 1767 to 94. So Bill's Gilbert & Co sextant may be a few years older than he thinks. But he may have later information which supercedes Taylor's account. She isn't always 100% reliable, though did a really useful job in assessing information in her two books about those practitioners. George. contact George Huxtable, at email@example.com or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. ----- Original Message ----- From: "Bill Morris"
To: Sent: Wednesday, June 16, 2010 8:13 AM Subject: [NavList] Do old sextants ever die? 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 ---------------------------------------------------------------- NavList message boards and member settings: www.fer3.com/NavList Members may optionally receive posts by email. To cancel email delivery, send a message to NoMail[at]fer3.com ----------------------------------------------------------------