H1, H2, H3 and H4
H1  , H2  , H3  and H4  are the four main timekeepers constructed by John Harrison in his attempt to find a means of keeping time accurately at sea. They were so named by Commander Rupert Gould when he re-discovered, cleaned and restored them in the 1920s and 30s. The first three are all large clocks developed by Harrison between the 1720s and 1760s. The fourth relatively small watch, developed in the 1760s, made a decisive break with the mechanics of the previous three. Harrison developed H1 with his brother James while living in Yorkshire, and the following three in London with his son William and in consultation with the Board of Longitude. The Board granted Harrison regular sums of money to develop the timekeepers and was involved in three trials of their use at sea. All four are now some of the most famous clocks in the world and are permanently on display at the Royal Museums Greenwich  .
When the Commissioners of Longitude and the coveted 'great reward' money were established by the Longitude Act [RGO 14/1:10r] in 1714, timekeepers were considered the least promising of the potential means for finding longitude at sea. Newton advised Parliament [BGN:1] that the lunar distance method was much more likely to succeed. The major issues were the effects that the motion of a ship, as well as the moisture and temperature change of the air had on any timekeeper on a long voyage. Harrison's success was in overcoming both of these issues in H1. He invented a gridiron bimetallic strip to control the expansion and contraction of a pendulum in different temperatures. He also used a system of connected spring balance weights to regulate the swing of the pendulum. His new grasshopper escapement similarly reduced friction. H1 was trialled for the Board of Longitude by John Harrison in 1736 on a voyage to Lisbon (the logbooks of which are in the archives at [ADM/L/C/82] and [ADM/L/T/23]). The success of the trial was largely responsible for the Board's first recorded meeting [RGO 14/5:3] in 1737. Harrison then developed H1's accuracy in H2 and H3 over 30 years, with support from the Board. The National Maritime Museum owns a detailed drawing of the back of H3 [ZAA0882]. Eventually, in the 1760s, work on his own personal watch suggested to Harrison that a watch could be the solution, and he developed H4. This used a completely different type of timekeeping with springs and jewelled pallets. H4 was sent on trials to Jamaica and Barbados with William Harrison (for which the logbooks [ADM/L/C/82] are in the archives).
Harrison's long work on developing these timekeepers created the first real administrative actions of the Board of Longitude. Like Harrison himself, therefore, they are mostly notable for their absence in the Board archives. Brief records in the early minutes [RGO 14/5], and in letters to the Admiralty [ADM/A/2528], record the granting of regular sums to support Harrison's work. The level of record-keeping increased in 1765 when H4 returned from the successful trial to Barbados, and Harrison began a long argument with the Commissioners over his entitlement to the longitude reward. These arguments appear briefly in the minutes [RGO 14/5], and in more detail in Harrison's own 'Journal' as well as papers belonging to commissioners, Lord Sandwich [SAN/F/2] and Viscount Barrington [BGN]. All of these show the role of the Astronomer Royal, Nevil Maskelyne, in assessing the watch, and Harrison's growing dislike of his authority. Maskelyne attended Harrison's disclosure [RGO 4/152] of the workings of H4, and was responsible for testing [RGO 4/311] all the timekeepers at the Greenwich Observatory. These conflicts ended in the 1770s, when George III encouraged Parliament to vote Harrison the equivalent of the great reward for H4. It is after this point that the timekeepers start to appear regularly [RGO 14/25] in the Board of Longitude archives, as they began the process of trialling copies made of H4 by Larcum Kendall, John Arnold, and Thomas Earnshaw. By the 1790s, Andrew Mackay could conclude [MKY/6:267] that timekeepers were the best means of finding longitude at sea.
History and Philosophy of Science
University of Cambridge