Grasshopper Escapement

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History

The grasshopper escapement was invented by John Harrison for use in his regulator clocks, and he also used it in his first three marine timekeepers, H1 - H3. However it was seldom used in other timepieces. Estimation of longitudinal position was a major problem in marine navigation: Newton argued that astronomical positioning could be used, but an easier theoretical possibility was accurate knowledge of time, relative to base (GMT). A large prize was on offer for an accurate clock and Harrison devoted his life to conceiving and building ultra-accurate clocks.

Measuring Longitude
The measurement of longitude is important both to cartography and to provide safe ocean navigation. Mariners and explorers for most of history struggled to determine precise longitude. Finding a method of determining exact longitude took centuries, resulting in the history of longitude recording the effort of some of the greatest scientific minds.
Latitude was calculated by observing with quadrant or astrolabe the inclination of the sun or of charted stars, but longitude presented no such manifest means of study. In 1714, motivated by a number of maritime disasters attributable to serious errors in reckoning position at sea, particularly such spectacular disasters as the Scilly naval disaster of 1707, which took Admiral Sir Cloudesley Shovell and his fleet, the British government established the Board of Longitude in 1714:

"The Discovery of the Longitude is of such Consequence to Great Britain for the safety of the Navy and Merchant Ships as well as for the improvement of Trade that for want thereof many Ships have been retarded in their voyages, and many lost..." [and there will be a Longitude Prize] "for such person or persons as shall discover the Longitude."

Determining Longitude using clocks or chronometers
Longitude at a point may be determined by calculating the time difference between that at its location and Coordinated Universal Time (UTC). Since there are 24 hours in a day and 360 degrees in a circle, the sun moves across the sky at a rate of 15 degrees per hour (360°/24 hours = 15° per hour). So if the time zone a person is in is three hours ahead of UTC then that person is near 45° longitude (3 hours × 15° per hour = 45°).  In order to perform this calculation, however, a person needs to have a chronometer (watch) set to UTC and needs to determine local time by solar observation.


Harrisons Chronometers
Harrison built five chronometers, two of which were tested at sea. His first, H-1, was not tested under the conditions that were required by the Board of Longitude. Instead, the Admiralty required that it travel to Lisbon and back. It performed excellently, but the perfectionist in Harrison prevented him from sending it on the required trial to the West Indies. He instead embarked on the construction of H-2. This chronometer never went to sea, and was immediately followed by H-3. Still not satisfied with his own work, Harrison produced H-4, which did get its sea trial and satisfied all the requirements for the Longitude Prize. However, he was not awarded the prize and was forced to fight for his reward.
Though the British Parliament rewarded John Harrison for his marine chronometer, H-5, in 1773, his chronometers were not to become standard. Chronometers such as those by Thomas Earnshaw were suitable for general nautical use by the end of the 18th century. However, they remained very expensive and the lunar distance method continued to be used for some decades.

Captain Cook, who discovered and charted the East Coast of Australia in 1770, carried a chronometer with him which he used to calculate Longitude, noting it as being a valuable aid because of its speed and ease of use. He continued to also use the "Lunar Distance" method which was far more complicated, time consuming and also needing a steady ship during the night, to measure longitude.