... I find a comparatively-cheap American-made plastic instrument,
the Davis Mark 15, entirely satisfactory, though you'll be
disappointed if you hope to locate your position with pinpoint
accuracy.
The Davis, for example, requires frequent adjustments to eliminate
index error (a problem shared by most plastic sextants), and, with
extended use, develops significant "lost motion" in the tangent
screw. Under ideal conditions -- a clear horizon (or a
well-shielded artificial horizon ashore) and a well-practiced
navigator -- you'll be doing well to get a position line within
two miles of your "true" or actual position....
Unfortunately, the Mark 15 will set you back about $100. Davis does
make a cheaper instrument, the Mark 3 (about $50 with a plastic
carrying case), but it relies on a vernier rather than a micrometer
drum, lacks a magnifying telescope, and has only a limited selection
of shades. I'd buy the Mark 15 or do without, though there is (or
was) a British-made plastic sextant, the EBBCO (for East Berks Boat
Co.) which comes well-recommended by both Coote and Schlereth. I've
never seen, let alone used, an EBBCO, but perhaps Bruce Longstaff or
another list member from across the Pond has had some experience with
one of these instruments.
When all is said and done, however, a sextant is simply a device for
measuring angles. If all you want to do is demonstrate the principles
of celestial navigation, you can make do with much simpler tools.
I've taken sights with everything from my geologist wife's Brunton
Pocket Transit to a plastic 6-inch rule, and you can do the same.
(I'm sure I don't have to remind you not to look directly at the sun
or its reflected image without some sort of protective shade, but
better safe than sorry, I suppose. I'm reminding you. And, by the
way, sunglasses -- even good sunglassess -- don't count.)
You can also recapitulate history and build your own cross-staff,
back-staff or octant (the immediate ancestor to the modern sextant).
If this idea interests you, see Dennis Fisher's _Latitude Hooks and
Azimuth Rings_ for plans and construction tips.
Good luck!
Forrest Brownell
South Colton NY
forrest@slic.com
PS Celestaire, a Wichita, Kansas, company, has a good selection of
navigation instruments, books and other paraphernalia. Their
well-implemented on-line catalog <http://celestaire.com> is worth a
visit.