MARINE CHRONOMETERS

The marine chronometer was developed in the XVIIIth century to help compute longitudes at sea.
While the measuring of latitudes had never presented sailors with too many problems, it was more difficult for them to compute longitudes with the same accuracy. In the first case it was a question of determining the elevation of the Sun on the horizon at midday, the moment of its highest elevation on the meridian, or of a known star (usually the North Star) at night-time, all of which could be done using astrolabes or quadrants. In the second case, however, it was a question more of calculating the moment when a celestial object passed across the local meridian and then, by means of nautical ephemerides, working out the difference in relation to the transit time of the same object across a known meridian (for example, that of the port of departure). It became important therefore to know the time with a much greater degree of precision than had been possible with the instruments traditionally used for telling the time, such as the clepsydra: an error of just two minutes could throw the ship off course by 30 miles.
This problem assumed enormous importance during the great ocean crossings of the XVIth and XVIIth centuries.
One of the first people to tackle the problem was Huygens who, in 1656, tried to use the mechanism of the pendulum. If the system gave excellent results on land it proved unrealistic at sea because of the constant moving of the ship which interfered with the movement of the pendulum.
In 1675 the Royal Observatory was established at Greenwich "to measure the long-desired longitude of places". When in 1714 the flagship of sir Cloudesley Shovell was shipwrecked off the Scilly Isles - when he thought he was sailing in the Channel, more than 100 miles away - the English Admiralty was forced to set up a "Longitude Committee". It was this latter which drew up the celebrated Longitude Act that offered a prize of £20.000 (more than £5 million today) to the person who invented a clock with a margin of error of less than two minutes on a return voyage to the West Indies.
The main efforts focused on protecting the balance-wheel from clockwork defects. In this way the verge (John Harrison) and spring (Pierre Le Roy) escapements were invented. The gimbal, finally, brought stability and planarity to the whole thing.
John Harrison (1693-1776), a self-taught carpenter from Yorkshire, entered the competition in 1735 with the first model of a clock (weighing more than thirty kilos) that he would work on for over 30 years. In 1761, Harrison’s son undertook a trial voyage to Jamaica taking with him the so called "Harrison N.4", the fourth prototype developed from the original. After 81 days sailing the chronometer was only five seconds late! A copy of this model, built by Larcum Kendall, was then used by James Cook on his second voyage, which lasted from 1772 to 1775: after three years sailing only seven minutes fourty-five seconds had been lost.
In Harrison’s chronometer the movement was controlled by two balance-wheels, connected by a spring, which compensated for temperature change by means of brass and steel bars. The main spring was partially recharged every seven seconds by a system of "power keeping" to avoid the clock slowing down during this process. The temperature compensation system and self-lubricating bearings represented a brilliant example of applied physics.
Although Harrison had managed therefore to solve the centuries old problem of computing longitude at sea, the Admiralty refused to hand over the whole prize-money and the intervention of George III was needed to pay part of the prize to the now eighty-year-old inventor of the marine chronometer.
In about the same period, in fact, the German Johann Tobias Mayer (1723-1762), superintendent at the Gottingen Observatory, had drawn up the first series of lunar tables accurate enough to act as basis for the "method of lunar distances" designed to establish longitude at sea. The method consisted of measuring the distance from the Moon of pre-established stars or planets; this distance measured at local time and then compared with the one recorded in the tables meant it was possible to obtain the time of the fundamental meridian to which corresponded the same lunar distance. The difference between the two times gave the longitude sought for.
In 1765 the British Admiralty granted part of the prize-money - £3.000 - to Mayer’s widow, thus recognizing the German astronomer, as well as Harrison, as inventor of a practical and reliable method for measuring longitude at sea.