NavList:

G'day Frank.

I suppose "endless" tangent screw distinguishes it from the tangent screw of a vernier sextant which allowed adjustment over only a limited range. There were however vernier sextants with endless tangent screws, the well known Heath and Co version and, less well-known, a Hughes and Son one from the 1920s that lacked only the correct pitch and a micrometer drum to be a proper micrometer sextant (I have illustrated these in a blog post at www.sextantbook.com).

The Smithsonian is not to blame for the term. I have a Mark II made by Ajax Engineering of Chicago with a certificate from the US Naval Observatory dated 7/7/1943 and under the heading CLASS is given "E.T.S." US naval sextants prior to the Mark II seem to have been Brandis vernier instruments, but manufacture of these ceased in 1932. I suspect that a lot of navigators bought their own from European makers.

Bill Morris
Pukenui
New Zealand
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