Clock-work, however, was his ruling taste as a workman, for many years, and he appears to have set before him as a task the making of a clock that should surpass all others. He says in one of his pamphlets that, in the year 1726, when he was thirty-three years of age, he finished two large pendulum clocks which, being placed in different houses some distance apart, differed from each other only one second in a month. He also says that one of his clocks, which he kept for his own use, the going of which he compared with a fixed star, varied from the true time only one minute in ten years. Modern clock-makers are disposed to deride these extraordinary claims, particularly those of Paris and Switzerland. We know, however, that John Harrison was one of the most perfect workmen that ever lived, and I find it difficult to believe that a man whose works were so true could be false in his words. 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.