NavList:

My observations for the Troughton mystery device follow.

The device incorporates two methods of reading an arc divided to 20 arc minutes (20’) to a precision of one arc minute (1’): one by the method of diagonals; the other by vernier.

The device comprises two main components that I’ll call the limb and the index plate.

The limb is marked with an arc graduated from 15 degrees to the left of zero to 25 degrees to the right, with graduation marks at 5-degree intervals. (The angle between the 15-left and 25-right marks is 40 degrees—not 20 degrees as would be the case for a sextant arc so marked.) The arc is divided to 20’.

The limb is also marked with 21 equally spaced concentric circular arcs. These arcs divide diagonal lines between the smallest radius arc and the largest, into 20 segments of equal length. The lines are spaced at 20’ intervals along the arcs and are diagonal in the sense that that the ends of each line differ in angle by 20’ relative to the arc centers.

The index plate includes a vertical ruler for the diagonals, marked at 5’ intervals from 0 to 20’, and a vernier scale.

The vernier has 20 divisions equal to 19 divisions of the arc, so it’s a direct vernier; that is, it reads upward in the direction of increasing angle on the arc.

As the arc is graduated to 20’ and the vernier has 20 divisions, the vernier least count is 20’ / 20 = 1’.

The vernier is folded, with 10 divisions to the left of zero and 10 to the right. A folded vernier is initially read in the direction of increasing angle on the arc. If no match is found between a vernier graduation and an arc graduation when 10 is reached, the count is continued from the other 10, toward the index.

Having the arc graduated in positive directions each side of zero allows demonstration of use of the folded vernier with an initial reading direction to the left (for the 15 degree side) as well as to the right (for the 25 degree side).

The index plate is moved along the limb by rotating a worm gear of pitch equal to the length of 20’ on the arc, allowing the knob on the gear shaft to be marked to 20’ for one rotation.

The seller’s photos of the device show the vernier index slightly to the right of zero on the arc. The diagonal ruler and vernier show, not too clearly, a measurement of ~ 5’. 

Chuck V.