NavList:

Here is further explanation of the Vernier scale in the sextant photograph below. Each division of the arc is 20’ of arc.  Each division of the lower (Vernier) scale is 21’ of arc. The readings on the lower scale start at the star in the centre and increase to the right from 0 to 10 in ten 21’ divisions.  The scale then moves to the extreme left and increases to the right from 10 to 20.  You don’t see 20 marked, because it coincides with the star. 

Consider the case when the arm is measuring an exact multiple of 20’s.  The mark at the star will be lined up with the mark on the arc.  Move the arm 1’ to the left.  I.e. increase the sextant value by 1’.  The 1’ mark on the lower scale will now be lined up with a mark on the arc.  Move the arm a further 1’ to the left.  Now the 2’ mark on the lower scale will be lined up with a mark on the upper scale. 

After moving the arm 10’ to the left, both the 10’ marks to the lower scale will be lined up with a mark on the upper scale.  From now on you must look at the left-hand side of the lower scale.  Move the arm a further 1’ to the left. I.e. the arm will now have moved a total of 11’, and the 11’ mark on the lower scale will be lined up with a mark on the upper scale.  After moving the arm a total of 19’ to the left, the 19’ mark on the lower scale will be lined up with a mark on the upper scale.  Moving the arm a further 1’ to the left gets the mark at the star on the lower scale lined up with the next 20’ mark on the upper scale.   I’m not sure why a manufacture would wish to mark a Vernier scale in this way.  It’s not essential that the zero mark of the Vernier must be in line with the CL of the arm.  Perhaps the arm scale had already been marked in a set position, or perhaps they just thought it looked tidier. DaveP