That sounds a bit strange until one stops to think of the rotation of the earth once in twenty-four hours. If one could travel around the earth, from east to west, at a uniform rate in exactly twenty-four hours, he would find clocks and watches indicating the exact minute he started at every step of his journey; and the sun would remain steadily at the same height above the horizon, if he always kept to one parallel of latitude. His rate of speed would have to be about eighteen miles a minute, if he chose to travel along the equator, or to state this same thing in another way, when it is noon in New York, it is 11 A. M. in Chicago, 10 A. M. in Denver and 9 A. M. in San Francisco; it is also 1 P. M. several hundred miles out into the Atlantic; 2 P. M. still farther out; 5 P. M. in London; and so on. In other words, it is some one of all the moments of the twenty-four-hour day at the _same time_, but _the time that indicates each of these moments is different at different points_. Therefore, if you could find out the time at any point, and could compare it with the time at the place you had left, you would know just how far east or west you had come, but not how far north or south. Ascertaining the time was not difficult; at noon it would be shown by the sun. Nor was it difficult to compare the time provided one had an _accurate_ timepiece, but a watch that ran either fast or slow might mislead one by hundreds of miles. You can see how important it was that navigators have some means of _exactly_ measuring time. This was one of the points at which the great mainspring of civilization pressed hardest upon the brains of inventors and the hands of workmen. So, from the sixteenth century onward, the leading governments of Europe offered large rewards for a chronometer sufficiently accurate to determine longitude at sea. In England, Parliament offered twenty thousand pounds, or one hundred thousand dollars, for a time-keeper which, throughout a voyage to the West Indies, would give the longitude within thirty miles. This meant that it must keep time within a minute a month, or two seconds a day. Both Huyghens and Hooke somewhat naively attempted to make a pendulum clock keep time at sea; but imagine the action of a pendulum while a ship was rolling and tossing!