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 conditio