The Stadiometer is a rangefinder that looks like a telescope but which has no lenses. It has a leather-covered barrel with an inset panel that holds a conversion table of tube length against height. The silvered brass fittings include two draw tubes with a scale of length inscribed along them. Looking through the eyepiece, there are two pairs of parallel wires at right angles to each other (the narrower wires are used for larger distances).

To use the instrument, the mariner lines up the top and bottom of the ship observed with the two parallel wires and then reads off the length from the scale inscribed on the draw tubes. They would then use the table in the inset panel to work out the distance based on the known (or estimated) height of the ship observed.

Rangefinders were designed principally to determine how far away other ships were, either for traveling in convoy or when fighting an enemy ship. This type was invented by G. H. Blakey, who was a Master in the Royal Navy, and was only made by W. Heath of Devonport.