JOHN HARRISON (1693-1776), an eminent mechanician, was the sou of a carpenter, and was born at Faulby, near Pontefract, in Yorkshire, in the year 1693. Thence his father and family removed in 1700 to Barrow, in Lincolnshire. Young Harrison at first learned his father's trade, and worked at it for several years, at the same time occasionally making a little money by land-measuring and surveying. The strong bent of his mind, however, was towards mechanical pursuits; and this showed itself specially in endeavours to improve the construction of clocks and watches so as to render them more accurate measurers of time. He soon learned that, to enable a clock to keep accurate time, the pendulum must be so constructed as to preserve the distance between the point of suspension and centre of oscillation invariable, notwithstanding the expansion and contraction of the rod caused by changes of temperature. To accomplish this Harrison devised, in 1726, his ingenious "gridiron pendulum," which consists in having the bob suspended by a series of parallel rods, alternately of steeland brass,so arranged that the downward expansion of the steel rods from cbange of temperature is exactly compensated for by the upward expansion of the brass rods. This principle of compensation, modified to suit particular cases, is now applied to all good watches and chronometers. Another ingenious improvement in clockmaking devised by Harrison was his recoil escapement, which obviated the necessity of keeping the pallets well oiled. He was led to invent this, as he himself tells us, by having on one occasion had to go a long distance to set right a turret clock which had stopped simply from want of oil on the pallets. This escapement, although answering admirably the intended purpose, is rather too delicate to be adopted in ordinary practice. Harrison was also the first to employ the commonly used and effective form of " going ratchet," which is a spring arrangement for keeping the timepiece going at its usual rate during the interval of being wound up.





In Harrison's time the Government of the country had become fully alive to the necessity of determining more accurately the longitude at sea. For this purpose they passed an Act in 1714 offering rewards of £10,000, £15,000, and £20,000 to any who should construct chronometers that would determine the longitude within 60, 40, and 30 miles respectively. Harrison applied himself vigorously to the task, and in 1735 went to the Board of Longitude with a watch which he also showed to Halley, Graham, and others. Through their influence he was allowed to proceed in a king's ship to Lisbon to test it; and the result was so satisfactory that the commissioners gave him £500 to carry out further improvements. Harrison worked at the subject with the utmost perseverance, and, after making several watches, went up to London in 1761 with one which he considered very perfect. His son William was sent on a voyage to Jamaica to test it; and, on his return to Portsmouth in 1762, the watch was found to have lost only 1 minute 54J seconds. This was surprisingly accurate, as it determined the longitude within 18 miles, and Harrison claimed the full reward of £20,000. After some further trials £10,000 of it was paid to him in 1765, and the remainder in 1767, after he had written such a descriptiou of his instrument as would enable other artists to copy it. Harrison died in 1776, at the age of eighty-two. His want of early education was felt by him greatly throughout life. He was unfortunately never able to express his ideas clearly in writing, although in conversation he could give a very precise and exact account of his many intricate mechanical contrivances. He wrote a book entitled Description concerning such Mechanism as will afford a Nice or True Mensuration of Times.