EARLY THREE MIRROR BACK SIGHT OCTANT
 JESSE RAMSDEN
OCTANT

Ca 1755 – 1777


 

PRESENTED is a last half of the 18th C. mariner’s octant with
the mark of Jesse Ramsden on the arc. It was Ramsden who invented the dividing machine which allowed for the precision marking
of degrees on an instrument’s arc in 1775. This instrument is a smaller version of the Hadley octant in which the “double
reflecting” principal was introduced by Hadley to the Royal Society in London in 1731. Hadley began selling octants
shortly thereafter which had a vertical dimension of 17″ or more. The instrument is 15 3/4 inches on the vertical. It
has been recently polished and lacquered, and is in a remarkable well preserved condition considering its 234 year age
and use at sea. You may read the Condition Statement for details.

The cross member, which seems
to be original is unmarked. This is not as unusual as many instruments were made to allow the retailer or an owner to
imprint their name which frequently was not done

MARKINGS: Using a magnifier you can clearly see an anchor
between 45 and 50 degree position on the arc. The anchor is the well known mark of Jessie Ramsden, perhaps the most famous
of all English instrument makers. It was Ramsden who invented the dividing machine which allowed for the precision indexing
of degrees on an instrument’s arc.

JESSE RAMSDEN, ONE OF ENGLAND’S MOST IMPORTANT INSTRUMENT MAKER’S HISTORY: Jesse
Ramsden (1735-1800)

 
Ramsden was one of the most famous English scientific-instrument makers of the second half of the eighteenth century. In
1775, he invented a semi-automatic dividing machine, capable of marking graduated scales on sextants and other nautical and
astronomical instruments for his own instruments and provided the same to other mathematical instrument makers. He developed
improvements in the manufacture of sextants, theodolites, barometers, micrometers, and countless other devices. One of the
most important instruments built by him is the great astronomical circle of the Palermo Observatory. In 1786, elected fellow
of the Royal Society of London and, eight years later, member of the Imperial Academy of Saint Petersburg. In 1795, won the
Copley Medal, the highest scientific award of his time: the prize was established in 1736 by the Royal Society, thanks to
a bequest from Sir Godfrey Copley.

What can be clearly seen is the anchor’s stock and flukes and with less clarity,
an “F”  to the left of the stock and a “P” to its right.There were many instrument makers whose names began with
these letters, but most of them made telescopes, microscopes and chronometers. We could find only one who made octants. Peter
Feathers, a Scotsman who operated from High St. and Dock St. in Dundee from 1842 until 1872. In 1873 he took in his son as
a partner, but this is way too late to be making such a large octant. Therefore, the most likely maker of the arc is the most
obvious, Jessie Ramsden, and the shop that did the assembly of the frame and working parts is lost to history.

                      
Maker’s mark a “F” Anchor “P” between 50° & 45°

Marked in pencil in a European hand on the center ebony upright is “J 69 over
18 space 44”. The 18 44 is not written as a date. 
The evenness of the spacing and markings
on the arc suggest that this instrument was made by a superior instrument maker, one who would stand at the top of his
trade. It is the equal in quality of any instrument of this early period.
It has a early style Ebony frame, flat brass
index arm, graduated arc and vernier, and precisely machined bronze fittings.
The three
shade glass assembly is movable for placement at either the index mirror position or below at the secondary position where
the third mirror allows for the taking of “back sights”.
 
For details on how we dated this instrument, see below.

                 Old style vernier
              Rear
peep sight & index mirror

The metal fittings
are precisely machined, and makes this instrument a work of engineering excellence for so early a period.

DATING THE INSTRUMENT: The arc measures from – 5 to
plus 95 degrees on a Ebony frame which is in excellent condition. It is known that Mahogany went out of use in favor
of Ebony some time after 1750. The octant has a flat brass index arm measuring a long 15 1/2 inches, with a tangent
screw. Tangent screws began replacing finger clamps in the last quarter of the 18th Century.
The horizon
mirror is mounted on an adjustable turntable. Its base can be rotated by loosening a screw which locks the mirror’s position.
A third mirror is below it for taking back sights and is also adjustable. The ivory vernier scale, reads from right to
left from 20 to 0 arc minutes, a two hole peep sight with “flap”, and movable three sun shades which serve both mirrors on
the frame.
 

All of these features date the instrument after 1750 and before 1780,
likely Ca 1775. Some time after 1780, the index arm clamp was superseded by an adjustable tangent screw, which also helps
date this octant. See Peter Ifland’s comprehensive sextant work, “Taking the Stars”. particularly figure 64, page
57.

                 Front peep sight & horizon mirror
              Period
stair step keystone case

        
VIEW THE GALLERY BELOW FOR MORE PHOTOGRAPHS

CONDITION:
The instrument is in good condition
except for some minor faults. There are some small age splits on the frame, and the nuts holding the two peep sights
are missing. The graduated arc, and all its hardware are in working order, but the index arm becomes stiff at 15 degrees. All
three bronze legs are original. It has the ivory pad on back, but the original small ivory scribe is missing. The
three mirrors all have remnants of silvering, but show their age. The machinery has recently been polished and lacquered.

The Case is  the typical stair step of this
period. We believe it not to be original to this octant though the two have been together for almost as long as the octant
is old. We reach this conclusion because it is not of the same quality as the instrument, and the wood is unfamiliar. The
top has a long age crack from front to back that has been repaired by a piece of sheet copper that is nailed from the
inside. The upper piece has a short age crack from side to side. It also is slightly warped and does not register exactly
when closed. Otherwise it is strong and sturdy, and the octant fits as it should. There is no lock, but there is a period
hook and eye.

                         Instrument
in case. Note copper repairs to cover

All in all, a rare,
desirable, and special example of an early Hadley style octant
.

DIMENSIONS:

  • Arc reads – 5 to +95 degrees
  • Length of index arm 15 3/4″ long
  • Width of frame at arc 13 ” wide
  • Index mirror 49 x 32 mm
  • Horizon mirror – 24 x 20 mm
  • Back sight mirror – 24 x 20 mm
  • Three movable shade glasses
  • Two Peep sights
  • Weight 2 Lbs 4 oz

OUR QUALIFICATIONS: We are one of the few company’s still selling
navigation instruments that know anything about them. For purposes of judging whether Joel’s opinion counts, he
was the editor of the chapter on sextants of the 1977 Edition of “Bowditch”, The American Practical Navigator, NAVPUB 9; a
member of the U.S. Naval Academy Navigation Symposium Board, 1975 -1978; the author of a book on marine sextants, Cornell
Maritime Press,1975, and the founding president of Nautech Maritime Corporation which partnered with Tamaya of Japan in the
introduction of the MS 733 Spica, the MS 833, Jupiter, MS 933 Venus sextants and the famous NC-2 navigation computer, in the
U.S. market. Joel is also a retired Master Mariner, and held a U.S. Navy “D” Qualification as a Senior Skipper – Oceans.
From 1995 through 2000, he served as a Varsity Offshore Sailing Team coach at the U.S. Naval Academy.