1. This rule was made in Switzerland about 1890.
2. The rule is made of metal - probably nickel/silver.
3. The instructions (modified slightly to remove references to figure numbers) read:
The face of the stadia rule is very compact and convenient for stadia reductions. The log distances are marked on the rule; the log ½ sin 2a is on the slide and is marked with degrees of the vertical angle a; on the runner is a scale of log cos2 a, which is used for obtaining the horizontal distances.
The example quoted in the instructions is for an observed distance of 210 feet and a vertical angle of 6°. The difference in elevation is read against the star at the end of the slide. There are three stars on the slide any one of which will give the true reading provided the star is not off the end of the rule. On the runner the 0° mark is set against 210 on the rule, and under the 6° mark on the runner is found the reduced horizontal distance, 208. On the runner is a little projection the edge of which is opposite the zero-point of the runner scale. When this index is set at the proper distance reading, the zero of the horizontal scale is then in the correct position. It will be noticed that the scale of the slide is arranged so as to increase from right to left, which is contrary to the usual arrangement of a slide rule. On the lower side of the Kern rule is a scale giving curvature and refraction units in metric units.
4. Kern and Company was well known maker of surveying instruments.
My thanks to Len Walker for providing the image and the text of the instructions.
Front view
Detail - front left
Detail - front right
