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George,

Sorry old friend, the arc of Alex's sextant  is divided into six spaces,
i.e., respectively 10-20-30-40-50 & 60 seconds to make-up 1-minute; the
vernier divides each 10-second space on the arc into individual seconds;
thus, a 10-second vernier. You read the whole degree + whole minute +
whole 10-second increment to the right of the vernier 0-mark, then follow
the vernier to the left until you find an exact match between the vernier
and arc graduations and add on the additional seconds indicated.

In addition to 10 and 15 second verniers, I must correct myself to add a
20 second vernier which was also made, though uncommon. There were, as
far as I know, no vintage instruments intended to be read in tenths.

I have a several page dissertation on reading-off verniers which I would
be pleased to scan and send separately to anyone wishing to receive it.

Regards,

Henry

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