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Although the idea of using a Chronometer to calculate longitude dates from the 13th century, the Chronometer itself was not made until the 18th century by a skilled English clock-maker, John Harrison. John and his brother, James, were so successful at correcting the existing causes of inaccuracies in clocks that by 1726, they had manufactured two clocks which lost no more than 1 second a month. This was a remarkable achievement and advanced far beyond any existing technologies of that time.

Harrison Number 1 (H1)

Harrison's H1.gifThe first of Harrison’s series of five sea-clocks (chronometers) was completed in 1735. The major improvement was that the pendulum originally used in the clock was replaced by a balance spring with two 5 pound weights connected by brass arcs. When the clock was tilted or turned by the movement of the sea, the weights attached will balance the spring and any particular movement communicated to one balance will be automatically counteracted by an equal and opposite movement of its opposing counterpart. The chronometer weighed 72 pounds. When the chronometer was put to a sea trial, it was relatively successful, losing only 3 seconds in 24 hours. However, Harrison decided that H1 could not be further improved and abandoned working with it turning his attention instead to the design and manufacture of H2.

Harrison Number 2 (H2)

Harrison's H2.gifIn 1739 H2 was completed. H2 was tall and heavier but it took up less deck space. The main innovation in the mechanism of H2, one which Harrison used in all his subsequent longitude time-keepers was a remontoire. The remontoire mechanism ensures that the force on the escapement is constant, thus improving the accuracy of the clock.

Harrison Number 3 (H3)

Harrison's H3.gifBy 1741 John Harrison had commenced H3. His aim was to achieve a uniform running of the clock. H3 was fairly similar to H2 but it was slightly shorter, lighter and had circular balances instead of dumb-bell shapes. A bi-metallic curb was used to allow for variations in temperature. However, H3 had the serious drawback of being impossible to adjust without dismantling and re-assembling, which were long procedures.

  Harrison number 4 (H4)

Harrison's H4.gifAfter the creation of H3, Harrison immediately began work on H4, which was his most famous and important timekeeper. H4 was definitely a breakthrough as its diameter was only 5.25 inches. It has a totally different appearance and mechanism from its predecessors. Oil was used as lubricants and to minimize the problems of ageing oil, Harrison used wheels and pinions with a great number of teeth that increased the efficiency of the clock. The results of the sea trials for H4 were amazing as it only lost 5 seconds in 2 months. This corresponded to an error in longitude of only 1.25 minutes.

 Harrison’s final longitude time-keeper H5 had been completed in 1772 and was mechanically very similar to H4.