Part 26 (1/2)

”I meant once to find out all about chronometers; but before I got started something interrupted me and I forgot it. I wasn't much interested in them anyhow, I'm afraid.”

”And now you'd like a few points, eh?”

”Yes. I know I shall get a great deal better idea of them if you tell me,” was the reply.

”If you weren't an American and I a Scotchman, I should say you were an Irishman,” laughed his companion.

”Why?” demanded Christopher innocently.

”Because you sound as if you had kissed the Blarney Stone. Well, if you wish to learn about chronometers you have chosen a somewhat difficult subject. It leads pretty far, you see. However, I will do my best to give you at least a few facts about them. In the first place the earth actually revolves on its axis in twenty-three hours, fifty-six minutes, and four seconds. We commonly divide our day, however, into twenty-four hours and let it go at that. But astronomers reckon more accurately.

They call our day the solar day and instead of having a clock with twelve figures on it as we do, they use one with twenty-four.”

Christopher glanced up with a smile.

”Why be so fussy about things like minutes and seconds?”

”Because sometimes such things as minutes and seconds make a great deal of difference. You may remember that when we were talking of sundials I told you they were not exact timekeepers.”

”I do remember.”

”You see, we reckon our day by two counts: one of them begins at noon and goes on--one, two, three, four o'clock, etc.--up to midnight; the other begins at midnight and ends at noon.”

”That's simple enough. I get that all right.”

”Now people didn't always do that. There were other countries that planned their day differently. The ancient Babylonians, for instance, began their day at sunrise; the Athenians and Jews at sunset; and the Egyptians and Romans at midnight.”

”How funny! I thought that of course it had always been done as we do it,” confessed Christopher, with frank astonishment.

”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 n.o.body 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 lat.i.tude 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 s.h.i.+ps were built and adventurous persons began to sail the ocean both for trade and conquest, captains soon discovered the stars and the compa.s.s 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 20,000 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 bra.s.s screws from which he selected one that was to his liking.

”Now there was living at this period John Harrison, a Yorks.h.i.+re 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.”