John Harrison's greatest achievement , supported by Captain James Cook, was to convince seamen that time, accurate enough for deep sea navigation, could be determined at sea by mechanical means. After the successful trial in 1765 of his coach clock, H4, he was awarded approximately half of the prize money of the 1714 Queen Anne Longitude Act. King George III persuaded Parliament to give him further moneys to equate his reward for this achievement to the whole of the Longitude Prize. Whilst he was developing his timepiece, the Rev. Nevil Maskelyne, 5th Astronomer-Royal, was preparing the Nautical Almanac, which was first published in 1766 giving the geographical position of the heavenly bodies and a table of the equation of time. Simpler chronometers, developed by contemporaneous clock makers, together with the Nautical Almanac gave more accurate navigation, increasing profitable trading and safety at sea .
When John Harrison constructed his first pendulum clock in 1713 no clock, whether on land or at sea could maintain the accuracy required by the Queen Anne Longitude Act. By 1726 he had developed a pendulum clock which he considered was accurate to one second per month and more importantly, which he considered could be improved. He continued development of his pendulum clocks after he moved to London in 1737 and fortunately described his ideas in a pamphlet "Concerning Such Mechanism" published in 1775 when he was 82 years old.
Study of his known remaining clocks, extant documents, drawings and his portrait enables understanding of his ideas and innovations. His knowledge of contemporary mathematics and natural philosophy (science) was extensive and pre-empted much work that was published in the following century. His last known pendulum clock, now known as the Royal Astronomical Society Regulator (RAS) is one of the most important mechanical artefacts of the eighteenth century because of its logical design associated with his drawings and documents. He claimed that his ultimate clock would be accurate to one second in a hundred days. A full description of how he could have achieved this is given in my book."John Harrison's Contrivance".
