"...every great captain... became lost at sea despite the best available charts and compasses."
One way of calculating the difference between a longitude at sea and a known longitude (of Greenwich, say) was to ascertain the mean solar time on the ship, by astronomical observations, and compare it with the time at Greenwich. To this end a clock was needed that accurately kept Greenwich time despite being rocked back and forth by the ship. In 1714 the British government offered a £20,000 prize (about £1,000,000 today) to anyone who could find
longitude at sea to an accuracy of 0.5 degrees.
Yorkshireman John Harrison (1693-1776) decided that an accurate clock was the answer. He built his first marine chronometer in 1735. This spring-driven clock was regulated by two connected balances that oscillated in opposite directions, thus eliminating all the effects of the ship's motion. Intentional variations in the lengths of the balance springs also compensated for temperature changes.
Harrison's third chronometer (1759) had a bimetallic temperature compensator and a remontoire to ensure that the escapement driving force was constant. Harrison's fourth chronometer (1761) embodied all his improvements into a large "pocket" watch, about 5 inches (13 cm) in diameter.
This watch was carried to Jamaica on board HMS Deplore, in 1761. The clock error over the journey was about five seconds, equivalent to a longitude error of about one sixtieth of a degree. The ship's position was thus known to an accuracy of 1.5 miles (2 km). Harrison finally got all his prize money in 1773, and soon every ship was carrying his instrument.
