A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Brad Morris
Date: 2015 Oct 20, 01:58 -0400
I do not treat this minor discrepancy between Gary and myself as anything serious; nor as a reflection of the purported difficulty of reading a vernier.
There was a transcription error between reading the vernier and recording the value in the email. Attributable to either an unfamiliar instrument (yes, I counted the lines, then redid it using the values the lines represented to be sure) or to a simple typographical error that we all make from time to time.
If this were a familiar instrument, the lines would have immediately made sense to me. Then I would proceed as indicated in the last email to avoid arithmetic errors. Record the Nonius reading. Record the vernier offset as indicated by the matching lines. Add.
Here is the trick I use to find the matching line. Look at the nonius. The line is well AFTER the arc 50°10'. Now look at the vernier at about 4', the line is well BEFORE the arc line. So we know the matching line is somewhere between those to vernier graduations. Skipping from back and forth, one can see the vernier and arc lines converge right around 2'. So now look at the group to the left and the right of 2' and you will see that in fact they 'point' to 1'50". By point, I mean that the vernier lines are all before or all after the arc lines. So it becomes obvious which is the right line fairly rapidly. With practice and a familiar instrument, this is no where near the cumbersome process some authors would make it out to be. It is fast and accurate. Perhaps not as fast as a drum but there is no need to make this out to be a long slog thru the mud! ;-)
I thought it reasonable to point out the discrepancy, as newbies attempting to read the vernier might be thrown by the other value provided.