Caption:

This instrument is a specialised circular slide rule, on which magnetic heading, leeway, deviation, and variation may be entered and from which a true course may be read, without arithmetical calculation. Before the invention of these devices, magnetic compasses could indicate the 'magnetic' north pole but sailors had to use arithmetical formulae to calculate the 'true' north. The 'True Course Finder' performed these calculations automatically and made corrections for any leeway, deviation and variation. It was patented by W H Roberts.