Reading Section Practice 1

ABy the early eighteenth century, sailors could establish their latitude — their position north or south — with reasonable confidence by measuring the height of the sun at noon. Longitude, the east-west position, remained a matter of estimation. A navigator recorded his speed and heading and calculated where he ought to be, a method known as dead reckoning that accumulated error with every current and every gust of wind.

BThe consequences were not merely academic. In 1707 a British fleet returning from the Mediterranean struck the Isles of Scilly, and around two thousand men were lost. Parliament responded in 1714 with an unprecedented prize of £20,000 for a practical method of determining longitude at sea, and appointed a Board of Longitude to judge the entries.

CAstronomers already knew the theoretical answer. Longitude is a difference in time: if a navigator knows the hour at a reference place and the local hour where he stands, the gap between them gives his distance east or west. The reference hour was the problem. Pendulum clocks, accurate on land, were useless on a rolling deck, and the leading astronomical alternative required observing the moon against the stars and then performing four hours of calculation, in clear skies, with instruments the average ship's officer had never used.

DJohn Harrison, a carpenter from Lincolnshire with no formal scientific training, took the mechanical route. His early sea clocks were large and heavy but demonstrated that a timekeeper could resist the motion of a ship. The instrument that finally convinced sceptics, completed in 1759, was radically smaller: a watch of about thirteen centimetres in diameter. On a voyage to Jamaica it lost only a few seconds over eighty-one days.

ERecognition came slowly. The Board, several of whose members favoured the astronomical method, demanded repeated trials and eventually required Harrison to explain his mechanism to other craftsmen so that copies could be made. He received the greater part of the money only after the intervention of the king, and by then he was almost eighty. Within a generation, however, marine chronometers derived from his design were standard equipment, and dead reckoning had become a backup rather than a method.