John Harrison received for his improved chronometers, in 1749, the Copley Medal; and, thus encouraged by the Royal Society, and by the hope of sharing the reward of 20,000_l._ offered by Parliament for the discovery of the longitude, Harrison produced in 1758 a time-keeper, which was sent for trial on a voyage to Jamaica. After 161 days, the error of the instrument was only one minute five seconds, and the maker received from the nation 5000_l._ For other chronometers, subjected, with perfect success, to a trial in a voyage to Barbadoes, Harrison received 10,000_l._ more. Dr. Stukeley writes of this ingenious man: “I passed by Mr. Harrison’s house at Barrow, that excellent genius of clock-making, who bids fair for the golden prize for the discovery of the longitude. I saw his famous clock last winter at Mr. George Graham’s: the sweetness of its motion, the contrivances to take off friction, to defeat the lengthening and shortening of the pendulum through heat and cold, and to prevent the disturbance of motion by that of the ship, cannot be sufficiently admired.”—_Ms. Journal._[18] An exact measure of time is of the utmost importance to many of the sciences. Horology is indispensable to astronomy, in which the variation even of two or three seconds is of the greatest consequence. By means of a clock the Danish astronomer, Roemer, was enabled to discover that the eclipses of Jupiter’s satellites took place a few seconds later than he had calculated, when the earth was in that part of its orbit the farthest from Jupiter. Speculating on the cause of this phenomenon, he calculated that light was not propagated instantaneously, but took time to reach us; and, from calculations founded on this theory, light has been discovered to dart through space with a velocity of about 192,000 miles in a second: thus, the light of the sun takes eight minutes to reach the earth. Horology has also enabled us to discover that when the wind passes one mile per hour, it is scarcely perceptible; while at the rate of one hundred miles per hour it acquires sufficient force to tear up trees, and destroy the produce of the earth. And, without the aid of a seconds-clock, it would have been scarcely possible to ascertain that a cannon-ball flies at the rate of 600 feet in a second. The use of Chronometers in geography and navigation is well known; since it is only necessary to ascertain the exact difference in time between two places, to determine their distance east or west of each other. Graham applied the motion of a Clock showing sidereal time to make a telescope point in the direction of any particular star, even when before the horizon.