A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Bill Morris
Date: 2013 Jan 9, 14:35 -0800
It is a nice old instrument and the illustrious Captain Lecky thought a lot of his. It can be no earlier than 1826, when Troughton took on William Simms as a partner, so Frank Reed's estimate of around 1830 is probably as close as you're going to get for a date.
You mention worn numbers. As long as they and their associated divisions are readable, leave them alone. If they are not readable then you have to decide whether you want a museum piece or a working artefact. If you broke a shade or needed a new mirror for a modern sextant, you would not hesitate to replace them and you would have felt the same way in 1850, so I like my instruments to be working, intact and looking like they just came back from the instrument maker. Rather like clocks and chronometers, sextants would have had an occasional clean and lubrication too.
I don't understand Frank's reference to wear near zero as there is no reason for that to be the case with a vernier instrument and certainly is not true for the dozen or so vernier instruments that I have. You really do need to resist the temptation to polish the silver (or sometimes platinum) arc and vernier. The graduations are shallow and abrasive polish will eventually remove them. If the arc is tarnished I suggest you use some ammonia solution on a finger tip to remove the tarnish and then rinse with clean water. You can make the graduations more visible by wiping across them with something like artist's black oil paint. Captain Lecky recommended lamp black in sweet oil (olive oil), but we do not need to follow his recommendation slavishly. It is very unlikely that you would now be able to find someone with the skills, appreciation and equipment to re-divide the arc if the divisions have gone.
As to cleaning generally, if you are not all thumbs you could remove all the mirror, telescope and shades brackets from the frame, followed by the index arm, but do not disturb the index arm bearing (the bit that encloses the tapered shaft or journal). You can then wash it in a weak solution of household ammonia to which you have added a good squirt of dishwash liquid, using an old tooth brush to brush verdigris, dust and debris out of all the corners. Rinse in warm water and when it is dry, brush off remaining debris with a fine dry brush. Re-assemble with a dab of waterproof grease on the index arm bearing and a thin smear of clock oil on the arc and sliding block of the index arm clamp.
If you are more adventurous, you can dismantle the mirror, telescope and shades brackets (having first studied carefully how they are assembled)in order to clean out dust verdigris and debris. Grease the shades shafts on reasssembly. It would be a kindness to the next person to work on your sextant to put a touch of clock oil on the threads of the screws as you re-assemble.
Providing that you do not disturb the bearing, you will not have affected the accuracy of the instrument by dismantling it, but do not on any account be tempted to dismantle the frame itself. You can find a brief description of its structure on my web site (www.sextantbook.com) under "Evolution of sextant parts".
If the mirrors are in poor condition, I personally would replace them with modern ones, keeping the old ones in the box, though they may well not be the original ones anyway. Shades glasses can be cleaned with alcohol (but check first that the black lacquer which I think I can see is not soluble in alcohol). The lenses in the telescopes can be unscrewed if you need to get to both sides. Be aware that the achromatic objective lenses of the period were air spaced (later ones were cemented) and if you separate them from their cells for cleaning, make sure you get the orientation correct when you put them back. For cleaning lenses, use alcohol, wiping up and down and using a fresh part of the cloth for each stroke so you do not rub grit into the glass.
If you look at my website under "Chasing tenths of an arc minute" you will find a link to two papers I wrote on Jesse Ramsden's circular dividing engine. In one of them are photographs of a Ramsden-type dividing engine in the Science Museum, London. It was probably the very machine used to divide your sextant though its provinence is not known for certain.
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