In perfecting these amateur clocks he made a beautiful invention, the principle of which is still employed in other machines besides clock-work. Like George Graham, he observed that the chief cause of irregularity in a well-made clock was the varying length of the pendulum, which in warm weather expanded and became a little longer, and in cold weather became shorter. He remedied this by the invention of what is often called the gridiron pendulum, made of several bars of steel and brass, and so arranged as to neutralize and correct the tendency of the pendulum to vary in length. Brass is very sensitive to changes of temperature, steel much less so; and hence it is not difficult to arrange the pendulum so that the long exterior bars of steel shall very nearly curb the expansion and contraction of the shorter brass ones.
While he was thus perfecting himself in obscurity, the great world was in movement also, and it was even stimulating his labors, as well as giving them their direction.
The navigation of the ocean was increasing every year in importance, chiefly through the growth of the American colonies and the taste for the rich products of India. The art of navigation was still imperfect. In order that the captain of a ship at sea may know precisely where he is, he must know two things: how far he is from the equator, and how far he is from a certain known place, say Greenwich, Paris, Washington. Being sure of those two things, he can take his chart and mark upon it the precise spot where his ship is at a given moment. Then he knows how to steer, and all else that he needs to know in order to pursue his course with confidence.
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.
In 1761, when he was sixty-eight years of age, he wrote to the commissioners that he had completed a chronometer for trial, and requested them to test it on a voyage to the West Indies, under the care of his son William. His requests were granted. The success of the chronometer was wonderful. On arriving at Jamaica, the chronometer varied but four seconds from Greenwich time, and on returning to England the entire variation was a little short of two minutes; which was equivalent to a longitudinal variation of eighteen miles. The ship had been absent from Portsmouth one hundred and forty-seven days.
This signal triumph was won after forty years of labor and experiment. The commissioners demanding another trial, the watch was taken to Barbadoes, and, after an absence of a hundred and fifty-six days, showed a variation of only fifteen seconds. After other and very exacting tests, it was decided that John Harrison had fulfilled all the prescribed conditions, and he received accordingly the whole sum of twenty thousand pounds sterling.