When John Harrison was a young man, the art of navigation had so far advanced that the distance from the equator, or the latitude, could be ascertained with certainty by observation of the heavenly bodies. One great difficulty remained to be overcome--the finding of the longitude. This was done imperfectly by means of a watch which kept Greenwich time as near as possible. Every fine day the captain could ascertain by an observation of the sun just when it was twelve o'clock. If, on looking at this chronometer, he found that by Greenwich time it was quarter past two, he could at once ascertain his distance from Greenwich, or in other words, his longitude. But the terrible question was, how near right is the chronometer? A variation of a very few minutes would make a difference of more than a hundred miles. To this day, no perfect time-keeper has ever been made. From an early period, the governments of commercial nations were solicitous to find a way of determining the longitude that would be sufficiently correct. Thus, the King of Spain, in 1598, offered a reward of a thousand crowns to any one who should discover an approximately correct method. Soon after, the government of Holland offered ten thousand florins. In 1714 the English government took hold of the matter, and offered a series of dazzling prizes: Five thousand pounds for a chronometer that would enable a ship six months from home to get her longitude within sixty miles; seven thousand five hundred pounds, if within forty miles; ten thousand pounds if within thirty miles. Another clause of the bill offered a premium of twenty thousand pounds for the invention of any method whatever, by means of which the longitude could be determined within thirty miles. The bill appears to have been drawn somewhat carelessly; but the substance of it was sufficiently plain, namely, that the British Government was ready to make the fortune of any man who should enable navigators to make their way across the ocean in a straight line to their desired port. Two years after, the Regent of France offered a prize of a hundred thousand francs for the same object. All the world went to watch-making. John Harrison, stimulated by these offers to increased exertion, in the year 1736 presented himself at Greenwich with one of his wonderful clocks, provided with the gridiron pendulum, which he exhibited and explained to the commissioners. Perceiving the merit and beauty of his invention, they placed the clock on board a ship bound for Lisbon. This was subjecting a pendulum clock to a very unfair trial; but it corrected the ship's reckoning several miles. The commissioners now urged him to compete for the chronometer prize, and in order to enable him to do so they supplied him with money, from time to time, for twenty-four years. At length he produced his chronometer, about four inches in diameter, and so mounted as not to share the motion of the vessel.