Parallel ruler by W. H. HARLING, London, ca. 1920–1930.

W. H. HARLING, London, ca. 1920–1930.
Rolling parallel ruler for drafting and navigation.
Boxwood and brass.
Marked “HARLING / LONDON.”
Includes a drafting attachment and original wooden case.
Condition: exhibits patina, wear consistent with age and use, scuffs, and other signs of wear.
Measurements: 4 x 40 x 8.5 cm (case).

An interesting sliding parallel ruler made by W. H. Harling, one of the most renowned British firms of its time specializing in mathematical and precision drafting instruments. Founded in London in the mid-19th century, the company supplied instruments for architects, engineers, surveyors, and navigators for decades.
Made of boxwood and brass, the ruler features graduated scales along its edges and a central mechanism consisting of a longitudinal shaft protected by a cylindrical brass cover. The rollers located at the bottom allow the instrument to be moved across the paper without altering the ruler’s parallelism, making it possible to draw successive parallel lines quickly and accurately.

This system was particularly useful in technical drafting, cartography, and navigation, where it was necessary to transfer courses, alignments, and reference points onto plans and charts. The built-in graduations further expanded its capabilities as a measuring and drafting instrument.

This example retains its original wooden case, specifically designed to hold the ruler and its drawing accessory. Due to its careful construction and eminently functional nature, it stands as a fascinating testament to British scientific and technical instrumentation from the early decades of the 20th century.