![]() The gravity of the heavy balance should be proportional to its radius squared while the other one would have a linearly increasing weight with its radius – ideally, both should rotate at the same speed around their common axis of rotation.Ĭompensating for their different weights was achieved by making them slightly different sizes and placing them slightly out of balance with each other. He also made a pocket watch version but this didn’t work either and the reason for this became apparent when he examined it: The H4 Harrison Longitude Watch 1735-1759.ĭuring his experiments, John realized that there were different types of balances and that their specific gravities (weights) differed even though they had similar shapes. The third version, H3, also had two balances placed back-to-back with the same balance springs to cancel out errors that came from temperature changes. It too used two clocks set at different times so that sailors could use them to determine their distance from homeport (the time difference between London and New York) but this model proved even more fragile than its predecessor and did not stand up well to prolonged sea travel. On his second attempt, he called his invention H2. The H2 used two clocks set at different times Harrison was finally ready to test his invention on an Atlantic crossing, but it failed after 6 weeks because saltwater had got inside and ruined some of its wooden components. There are deep-sunken subdials displaying, on one side of the clock, time at home port (in this case London) and on the other side time at sea – here the scale is in nautical miles.īoth scales automatically compensate for the changing length of a nautical mile as one gets closer to either pole. The brass dial shows the hours and minutes in Roman numerals – these are engraved into the metal that forms the chapter ring around which other rotating inner rings move to display seconds, calendar date, etc.Īll this is surrounded by an iron cage/bezel to protect it from knocks. It has an iron frame for rigidity so that it can withstand the immense forces placed upon it at sea and because magnets interfere with its delicate mechanism. This is a large watch about 43mm across not including the crown. Harrison’s first attempt was above, called H1. Undaunted, he kept working on his design. He also invented the bimetallic strip thermometer (used in thermostats). The first clocks made by John Harrison could keep time to within about 10 seconds per day, which is reasonably good but not good enough for sea-timekeeping. Note: If you want to understand how longitude can be measured by using a marine clock and the position of the Sun and the stars, read on…If not – or if you would rather watch a video take a look at this one. The prize was finally awarded 263 years later in 1978. Several solutions were suggested for this problem and many working prototypes were made but none was able to overcome the problems of size, accuracy, affordability, and reliability. The Longitude Prize funded by HM Government in 1714 offered £20 000 (over 5 million pounds todaY) for anyone who could solve this problem. Not surprisingly this was not good for trade! We are all familiar with navigational terms such as latitude – distance north or south of the equator – but longitude was more difficult because there was no way of accurately measuring it until John Harrison’s invention. Captains might find themselves up to 50 miles away from where they expected their ship to be. The underlying problem is that, when the time according to the local clock and the time according to a reference clock are different, it becomes difficult or even impossible to determine which one of the two ships is in the right place. ![]() Longitude is defined as the angular distance between the point where you are standing and another reference point such as Greenwich Meridian or another place which is agreed as having longitude zero. ![]() The Board of Longitude rewarded him £20,000 (worth several million pounds today) although there was some controversy as to whether he deserved it: What difference would a longitude clock make? His marine chronometer, H1, was first tested in 1736, and improved versions went on to help Captain James Cook on his voyages of discovery and were used by Nelson to defeat Napoleon. The credit for inventing the marine chronometer, an essential piece of equipment that enabled accurate navigation at sea, must go to John Harrison. ![]()
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