CHARTING THE RIVER 117 



or star at any place could be compared with the time of merid- 

 ian passage at Greenwich, by reference to the position of Jupi- 

 ter's satellites. Unfortunately for Galileo's suggestion, though 

 this observation is perfectly feasible on shore with a firm and 

 level basis for astronomical instruments, it is not a practical 

 possibility aboard a ship at sea. The problem therefore became 

 quite simply that of carrying aboard ship a reliable clock. 



The clock used for timing the stars, sun, and planets could 

 not be an ordinary clock. The earth revolves once every 

 twenty-four hours, and the circumference is about 21,600 

 nautical miles, so that at the equator a mistake of one minute 

 would equal an error of as much as 15 miles. As far as land 

 clocks were concerned the problem was solved by Galileo 

 when he discovered the simple truth that a pendulum of a 

 given length always takes the same time to complete its swing. 

 Using this principle, Christian Huyghens in 1656 constructed 

 the first reasonably accurate clock. But the pendulum prin- 

 ciple, like most simple truths, is a good deal more complicated 

 than it first appeared; and it was soon discovered that the time 

 of swing, and therefore the speed of the clock, was different at 

 different parts of the earth and in different temperatures. 

 Moreover, the rolling and pitching of a ship interferes with 

 the movement of a pendulum, so that such a clock is unsuit- 

 able as a marine chronometer. For all this, in 1681 the pendu- 

 lum clock was used along with observations of the sun and 

 Jupiter's planets to make the first longitude determination in 

 the western Atlantic at Martinique and Guadeloupe in the 

 West Indies. It took three years to complete this observation! 

 The method was obviously unsuitable for shipboard use. 



The matter was brought to a head at the beginning of the 

 eighteenth century when an admiral and his fleet were lost on 

 account of a gross error in navigation. Sir Gloudesley Shovel, 

 returning from Gibraltar to England, ran ashore on the rocky 

 Scilly Isles, at a time when his navigators thought they were 



