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Geoffrey Kolbe pointed us to a website explanation of an "equiangulator",
and added-

| It may be of interest that the method of equal altitudes is probably
| the most precise method of determining a position using a "portable"
| optical device.
|
| The device - called a precision astrolabe in Europe and an
| 'equiangulator' in the United States - is quite simple. A telescope,
| a beam splitter and a pool of mercury. By noting the elapsed time
| between the moments when the reflected and direct image of a given
| celestial object were seen to coincide in the eyepiece, it was
| possible to determine position to great accuracy.

But that's not how the explanation reads, to me, in

http://www.aggregat456.com/2006/07/geodet.html

This is how it's described-

"To determine the longitude and latitude of a geodetic control point, a
geodet used an equiangulator to observe a series of stars "at a fixed and
invariable angle of 60 degrees." Looking through the eyepiece, the geodet
looked for two images formed by a passing star, one "produced by the light
of the observed star that falls on the upper side of the [equiangulator's]
prism and the other image is formed by the light reflected into the
underside of the prism from the artificial mercury horizon." At the moment
the two images would meet, a stopwatch was used to record the time of their
formation, and that measurement is calibrated with a coded signal from
Greenwich, England or the United States Naval Observatory in the District of
Columbia. "

From those words, it seems to me that there are not two moments to time for
each star, but just one, when the angle between its direct and reflected
images corresponds to a precise value set by a prism, of 60 degrees.
Presumably, by choosing stars at different azimuths (say, southeast and
southwest), and precisely timing the moment, by GMT, that each happens to
pass through an altitude of 60 degrees, then as long as the coordinates of
each star are precisely known, you get two position lines which can be
crossed. The only correction would be for refraction, which would be small
(and more important, precisely known) at such a high altitude. There's
nothing said, in that account, about measuring the time interval between two
equal altitudes for the one star, though there would be no objection to
doing so.

It looks like a simple operation which could be managed with very little
training: all it would need is the ability to tell one star from another.

George.

contact George Huxtable, at  george@hux.me.uk
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.


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