But how are you to determine your longitude? The pole-star, or sun, or any other star won’t help you, for as the earth is moving they keep shifting, and at one time or another appear exactly in the same position to everyone on the same parallel of latitude, as it is easy to see. The fact is that you are on a ball turning round. You know easily what latitude you are on, but you cannot tell your longitude unless you can tell how many hours and minutes you get to a position before Greenwich gets to the same position. If when a particular star got to Greenwich a gong were sounded which could be heard all over the earth, then of course, by seeing what stars were overhead, everyone would know their longitude at once. Perhaps by means of the new electric waves this will before long be done, and the Greenwich hours will be sounded all over the world for the use of mariners. But till this is accomplished all that can be done is to keep an accurate clock on board, so as always to give you Greenwich time. Early attempts were made to take a pendulum clock to sea, suspending it so as to avoid disturbance to its motion by the rocking of the ship. These proved vain. It therefore became desirable that a watch with a balance wheel should be contrived to go with a degree of accuracy in some respects comparable with the accuracy of a pendulum clock. To encourage inventors an Act of Parliament was passed in the thirteenth year of Queen Anne’s reign (chapter xv.) (1713) promising a reward of £20,000 to anyone who would invent a method of finding the longitude at sea true to half a degree—that is, true to thirty geographical miles. If the finding of the longitude were to be accomplished by the invention of an accurate watch, then this involved the use of a watch that should not, in several months’ going, have an error of more than two minutes, which is the time which the earth takes to turn through half a degree of longitude. This was the problem which John Harrison, a carpenter, of Yorkshire, made it his life business to solve. His efforts lasted over forty years, but at the end he succeeded in winning the prize. These instruments have been much improved by subsequent inventors, and have resulted in the construction of the modern ship’s chronometer, a large watch about six inches in diameter, mounted on axles, in a mahogany box. Several of these are taken to sea by every ship. The peculiarity of the chronometer is its escapement. Let _A B_ be the scape wheel, and _C D_ a small lever attached to _C_, the pivot on which the balance wheel and spring is fastened. Let _E G_ be a lever, with a tooth _F_ which engages the teeth of the scape wheel and prevents it moving round. Let _H_ be a spring holding the lever _E G_ up to its work. [Illustration: FIG. 66.]