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Ken Muldrew wrote, responding to a posting from Jim Hickey-

| > My personal preference, assuming I had only a choice of a graphical method
| > like Margetts, a tabular method or a slide rule type method. I would choose
| > the slide rule method first, graphical second and the tabular last. Clearly
| > I would be out in left field compared to the norm.
|
| Compared to the 1790 norm, anyway. I would choose the same order as you
| (if there was a slide rule type method to choose from).

====================

Well, there was, it seems. One such method appears to have been available, from Margetts himself.

This is what Eva Taylor had to say about Margetts, in "The mathematical 
practitioners of Hanoverian England, 1714-1840".

868 Margetts, George, M.I.M. (1789-1803)  [presumably, these were they dates 
between which he flourished]. 42, Penton Srtreet,
Islington ~(1789),  London ; 21 King Street, Cheapside, London (1792) 3 Cheapside, London (1801).

1789-90. Horary & Longitude Tables. These were accompanied by engraved plates, 
or Rotulas which allowed graphical solutions of
nautical problems:
1. A Logarithmic sexagesimal Rotula for giving proportional parts.
2. A Rotula containing the Line of Numbers, Sines, Tangents,  etc., equivalent to a 6-foot Gunter's Rule.
3. A Decimal Rotula for Mensuration & Gauging.

1790-3. Tables for correcting the effect of Parallax and Refraction.
Thomas Lynn [1009] said he had used Margett's tables for thirty years at sea, 
and Mackay [866] praised his 4-foot slide rule for
navigation, while Patrick Kelly [736] describes his rotulas, which could also 
be used for solving surveying problems.

The rest of Taylor's entry refers to Margetts' timepieces.

Taylor is frustratingly vague about what Margett's slide rule actually did.

There were other such instruments, it seems, I copied to the list a similar 
account from Taylor, about David Thomson and his brass
instrument for correcting lunar distances,  under threadname "Lunars: 
Thomson's tables", on 19 April. Again, it's short on detail.

There may be an interesting project awaiting someone, to search out any such 
instruments in museums, and discover how they were
used.

George.

============

contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

   

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