This instrument is used to determine the distance of another ship and so was particularly useful for ships sailing in convoy to ensure that they remained far enough apart.

The distance meter has a black metal body with a bakelite handle and a rubber eyepiece cup. There is a glass folding cover for the two scales on the side. The top scale is marked ‘CABLES’ and has a non-linear scale from 1 to 10. The bottom scale is marked ‘HEIGHT IN FEET’ and has a non-linear scale from 30 to 150.

The meter works on the double image principle. To use it, the observer sets the height scale to the known height of the ship being observed, then looks though the eyepiece and turns the knob on the front until the two images seen through the eyepiece coincide. The distance of the ship from the observer is then indicated on the scale marked ‘CABLES’.