During his lifetime, John Harrison created five timepieces designed to aid in the calculation of longitude at sea. While their external shape and size varied greatly, they shared common characteristics that made them radically different from earlier clocks and watches, largely because their internal mechanisms compensated for a number of technical difficulties that interfered with the ability of a mechanical timekeeper to function regularly. Among these were problems of friction, damage from natural elements (especially changes in temperature), and irregularities caused by the use of a metal spring that must be wound.

By the time Harrison came to design H4, a watch measuring 5.5 inches in diameter, he had managed to find solutions for most of these problems. When developing his larger clocks, he had perfected a system of balances that replaced the conventional pendulum, which had to be maintained in an upright position to work accurately. The new balances permitted the clock to continue to operate accurately regardless of its position, an important innovation since the rocking motion of a ship at sea made it impossible to keep the clock vertical at all times. In creating H4, Harrison transferred this idea to a set of balances that functioned in a similar fashion inside the smaller device. He solved the problem caused by the expansion and contraction of metal parts by developing a bimetallic strip, which helped regulate the movement of the watch. Because the two metals expand and contract at different rates in different temperatures, one counteracts the other; this allowed the watch to keep time accurately despite fluctuations in temperature. Harrison also developed a mechanism for transferring the power of the mainspring, which could fluctuate as it gradually lost tension after being wound, through a secondary spring that was constantly rewound so that it applied a constant force to the movement of the clock's hands.

Harrison's chronometers proved that it was possible to create a device that could keep time at sea with sufficient accuracy to make reliable calculations of longitude at any point on the globe. His combination of specially designed balances, his employment of a bimetallic strip to compensate for expansion of metals at different temperatures, and his development of a mechanism to ensure that time was kept accurately while his chronometers were being rewound were revolutionary ideas that paved the way for the production of chronometers that would allow ships to sail with greater assurance of their position.

The first real vindication for Harrison's efforts was accomplished before he died. Captain James Cook departed England in 1772 for a South Seas voyage, taking along the copy of Harrison's chronometer made by Kendall. As Cook reported, that device proved exceptionally valuable for determining longitude and charting the lands visited by him in his three-year cruise. While many improvements were made in the decades following Harrison's death, several devices modeled on his designs were constructed for general use. Not only did these provide a means of calculating a ship's position, but, as Cook had shown on his voyage, they also were instrumental in helping mariners revise charts of the ocean and its land masses, especially islands located at remote places around the world. As a consequence, the work Harrison did in developing his sea clocks paved the way for the expansion of both commercial trade and exploration that occurred during the nineteenth century and beyond.

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