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On Bruce Bauer's "The Sextant Handbook".

I spent an enjoyable hour or so today reading this book at our local
library, and I have come to some conclusions.

First, that it does its job quite superbly, as it claims; a sort-of "owner's
manual" for sextants, with lots of useful and very practical information. I
really don't understand how I've missed out on it all these years. It
certainly calls for a place on my shelves and tonight I have ordered up a
copy.

But second, I would say that the historical side was not Bauer's forte, and
his "short history" chapter has a number of failings, on top of those that
Nicalas has pointed to. Some are just minor niggles of mine, that are not
worth listing.

One is an omission. Unless I've missed it elsewhere, he doesn't explain the
vital point about a 2-mirror instrument, which is this. As long as the two
objects being measured (such as Sun and horizon) are brought into the field
of view, it really hardly matters exactly which way the instrument itself is
aimed. The two move together. With an instrument with no mirrors (e g
backstaff) or 1 mirror (Hooke proposal,  Halley proposal, speigelboog) not
only do the Sun (image or shadow) and the horizon have to be aligned, by so
does the body of the instrument have to be aligned with both, to good
precision. Which doesn't detract, much,  from use on land, but makes them
difficult to use at sea. I think Bauer misses a trick by failing to point
that out.

And then he touches the Newton-Halley story we've been discussing on
navlist. As I've written in another posting, Bauer's is the first text I
know of to print the Royal Society Journal Book entry about Newton's
instrument in full, with its vital Halley reference. However, Bauer goes on
to say that "Halley had returned from a voyage to Brazil, during which he
evaluated an instrument designed by Dr. Robert Hooke, a rival of Newton.
Hooke's instrument used a mirror, but did not incorporate the double
reflecting principle. It did not work very well". It's quite true that the
only Hooke instrument we know about had only a single mirror, so would have
worked badly at sea. But Bauer doesn't say on what he based his story about
Halley evaluating that instrument on his Brazil trip, so we have no way of
checking it. I haven't seen that claimed elsewhere, and it seems most
unlikely to me.

And then he reproduces that familiar Royal Society engraving. He can hardly
be blamed for doing so, almost everyone's book shows the same thing,
completely inauthentic though it may be. But Bauer takes it to extremes, by
taking the dimension given for Newton's telescope (3 or 4 feet) literally,
scales the brass plate accordingly without just cause, and places the
enormous, impractical result in a shipboard setting that is entirely
imaginary (see attachment).

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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