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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  george@hux.me.uk
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

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