Sine rule sine of angle = opposite / hypotenuse (SOH)
There are three basic trig rules, the sine rule, the cosine rule, and the tangent rule.
This instrument consists of two copper rules of equal length, where the thicker and wider, vertical sine rule has a hole and on which is attached the smaller, horizontal versine rule (or sahm, lit.
We move the alidade until a given star is seen through both of its sights, then the sine rule is moved to touch the alidade on the point that gives the sine of the star's altitude.
Based on this length and dimensions for the rules found earlier in this text, an alidade and versine rules of 1 fg x 1 fg x 3 cb is about 14 kg each; the sine rule, with estimated 1.
Given the sliding sine rule and the purpose of the catgut to prevent the sine rule from falling off the versine rule, the only fixed point can be the end of the versine rule where also the alidade is attached.