"Not at all. Our present system of time-keeping has been evolved out of the past and, like many other such heirlooms, is the result of a vast amount of study. Centuries ago nobody knew how to reckon time or what to reckon it by. Some computed it by the sun and had what is known as the solar day--a span of twenty-four hours; others figured it by the moon and got a lunar day of twenty-four hours and fifty minutes; while still others resorted to the stars or constellations and reached a result known as sidereal time, a day of twenty-three hours, fifty-six minutes. Now you see there is quite a bit of difference in these various reckonings. The difference might not matter so much on land, but when one is at sea and has to compute latitude and longitude, it matters a vast deal." "Oh!" A light of understanding was slowly dawning on the boy. "Now," went on McPhearson, "apparent solar time is dependent on the motion of the sun and is shown by the sundial; mean solar time, on the other hand, is shown by a correct clock; and the difference between the two--or the difference between apparent time and mean time is technically known as the equation of time, and is set forth in a nautical almanac published by the government." McPhearson waited a moment. "And that's what mariners use?" "Yes." "Then," hazarded Christopher after a moment's thought, "there really is exact time and common time." "Broadly speaking, yes," acquiesced McPhearson. "Or in other words there is time scientifically measured and time that is measured by man-made laws. The difference, as I told you, is of more importance to astronomers and mariners than to anybody else; and yet the puzzle for many centuries balked those who sought to establish a perfect system of time-keeping. As better ships were built and adventurous persons began to sail the ocean both for trade and conquest, captains soon discovered the stars and the compass could not be relied upon to furnish them the reliable information they needed in locating their position. Therefore, about 1713 England offered a prize of L20,000 to any one who should invent a timekeeper sufficiently accurate to enable navigators to ascertain from it longitude at sea." The Scotchman paused to take from his table a box of tiny brass screws from which he selected one that was to his liking. "Now there was living at this period John Harrison, a Yorkshire clockmaker, who although quite a young man had made a clock with wooden works into which he had put a gridiron pendulum--a device he had thought out to overcome the difficulties resulting from atmospheric conditions. This clock was so skillfully adjusted that it did not vary a second a month. So you can see that despite the fact Harrison was not a member of the Clockmakers' Company he was certainly qualified to be." "And did he go after the prize money?"