John Harrison Simulacrum

The Life

John Harrison was born in 1693 in Yorkshire, the son of a village carpenter, and had almost no formal education. He taught himself clockmaking, built his first long-case clock at twenty, and spent most of his working life in pursuit of a single problem: the determination of longitude at sea.

The British Longitude Act of 1714 had offered a prize of £20,000 — an enormous sum — to any person who could devise a means of determining a ship's longitude to within half a degree on a voyage to the West Indies. The astronomical approaches favoured by the scientific establishment were theoretically elegant but practically unreliable. Harrison took a different line: build a marine chronometer so accurate that it would keep Greenwich time through a transatlantic voyage, and longitude could be read directly from the difference between Greenwich time and local noon.

Between 1735 and 1761 he built four chronometers, each better than the last. The fourth, H4, completed when Harrison was sixty-eight, tested on the voyage to Jamaica in 1761–62 and with a different captain in 1764, kept Greenwich time within a few seconds over weeks at sea. The Board of Longitude, unwilling to credit a workshop clockmaker over its own astronomer members, withheld the prize for more than a decade. Harrison received the full sum only in 1773, three years before his death, after personal intervention by George III.

The Thought

Harrison's insight was mechanical rather than philosophical, but it was a mechanical insight of the first order. A portable clock aboard a ship had to keep time despite the ship's pitching, rolling, and yawing; despite temperature variations across the latitudes of a voyage; despite the humidity of salt air; and despite the inevitable friction of its own moving parts. Harrison's solutions included the gridiron pendulum (which compensated for temperature by combining metals of different coefficients of expansion), the grasshopper escapement (which reduced friction dramatically), and eventually, in H4, a design based on a fast-oscillating balance wheel rather than a pendulum. Each of his four chronometers represented a distinct approach to the problem; H4 represented the abandonment of an approach he had pursued for thirty years and the invention of a new one.

The Legacy

Once the marine chronometer was established, longitude at sea became a routine determination, and the combination of reliable longitude with Mercator's rhumb-line projection gave navigators the two tools they had lacked for oceanic sailing. The consequences for the British maritime empire, the commercial shipping industry, and oceanic exploration were immense. Harrison's chronometers are preserved at the Royal Observatory at Greenwich, and H4, still running when taken out for demonstration, remains as convincing as it was in 1764.