NavList:

Are they 3s or are they 5s?  They look like 3s, but the figures go ‘thingy zero’ to ‘thingy thingy’, and there are five divisions in between, so they must be 5s.  The arc scale is in degrees and 1/6degrees and the pointer shows 50° + 1/6° + a bit or 50° + 10’+ a bit. The Vernier is in minutes and 1/6 minutes i.e. 10 seconds.   The line up is between 1'+3/6' and 1'+5/6'; let’s say 1'+4/6'or 1’40”.  Therefore, the total reading is 50°+10’+1’40” i.e. 50° 11’40”. The confusion between 30", 40", and 50" does show what a tiny amount a second of arc actually is.  Fortunately, many people come to sextants with previous experience of verniers.  A technical education before the days of digital readouts meant that you kind of got used to verniers.  As well as vernier callipers, I've seen them on mercury barometers and even a periscopic sextant mount for airborne compass swinging, albeit as part of a flight test on a specialist navigation course.  DaveP