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النشر الإلكتروني

CHAPTER X.

TIME.

"Old Time, in whose bank we deposit our notes, Is a miser who always wants guineas for groats; He keeps all his customers still in arrears

By lending them minutes and charging them years.”

-Holmes.

Importance of

In this busy age, when more progress is made in a minute than was formerly made in an hour, and the exacting demands upon men in all walks of life make accurate time. them more chary of hours than their forefathers were of days, the importance of accurate time is realized as never before. The piercing whistle of the factory or machine shop wakes the echoes of the early morning. at the exact moment when some steady clock reads seven, and hundreds of working people take their places promptly, to begin the day's toil. The railroad conductor, with pocket chronometer in his hand, stands beside the palatial through train, while the engineer holds the panting locomotive in check, till the signal is given to open the throttle and speed the waiting passengers on their way.

"Thirty seconds too late," says the depot clock, as the belated traveler hurries to the platform, only to Tardy people. find that the train has pulled out. "Our clock at home was five minutes slow," says the blushing schoolgirl, when called to account for her tardiness. "The school clock must be a minute and a half too fast," says the boy who played marbles two minutes too long. The

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A wreck.

Small fractions.

business man paces impatiently to and fro in his office, waiting for friends who were to come precisely at three. The electric car has just gone by, and the mistress of the house, arrayed for an afternoon's shopping, stands on her doorstep in a pet; the kitchen clock was two minutes slow. The careful mariner, feeling his way along the coast, through a fog, feels a shock which shows that the good ship has struck a rock. The trusted chronometer has gone wrong, and the ship must go down in the seething floods.

Scientists dispute about tenths of seconds, quibble over hundredths, and take still smaller fractions into account, while the world wonders how they contrive to measure intervals of time so minute.

Though all the daily doings of the civilized world are Standard time. governed to a large extent by the timekeepers which are to be found everywhere, few stop to inquire into the authoritative source of standard time, and the methods of its dissemination. People generally have vague notions that astronomers observe the sun when it is on the meridian, regulate their clocks accordingly, and then telegraph the time about for the benefit of railroads and jewelers.

Time observations.

Let us go to the bottom of this matter, by visiting an observatory and seeing just what the astronomer does; we must not go at mid-day, for he does not use the sun to get time by. In the evening we may find him at work, and fortunate shall we be if he permits us to sit down in the room where he is observing, and silently watch his operations. In the center of the dimly lighted room is the meridian circle, which we have described in Chapter IX. The roof shutters have been opened, and we may see the stars trooping past on their way to the western horizon. On a table near the

instrument stands a chronometer, ticking off each half second; by its side lies a book, containing a list of stars. The book gives the right ascension and declination of each star. The astronomer glances at his chronometer and sees that its reading is about 8hr. 53min. In the list he finds a star whose right ascension is ghr. 56min. 4.93 sec. The star therefore will cross his meridian about 8hr. 56min., and will come into the field of view of his instrument a few seconds before that time.

The circle is

Looking at the declination he mentally figures out the reading of the silver circle, when the telescope has the set. proper slant to the horizon. In a minute he has turned the telescope on its horizontal axis till the circle has the proper reading, and has applied his eye at the eyepiece. Faint stars come drifting through the field of view, shying past the golden spider-webs, as if they wished to escape from the astronomer's gaze as quickly as possible; but he pays no attention to them.

In a short time the expected bright star appears on the edge of the field of view, glowing like a little sun. The observer glances quickly at the chronometer, and begins counting the readings of the second-hand; "four, half, five, half, six, half," he says to himself, as he resumes his place at the eyepiece. The star moves onward; it has leaped across the first spider-web, and the astronomer hurriedly writes in his note-book the figures 13.1.

The star comes.

Accurate esti

He has estimated that the star crossed the first spiderweb one tenth of a second after the chronometer ticked mates. the thirteenth second of some minute. Hurriedly glancing at the chronometer's face he again counts, and after a few seconds he makes another record, perchance 24.7. Thus he continues till the star has crossed the last spider-web; having gotten the seconds and fractions of

a second as correctly as he can, he writes down the minute and the hour more leisurely. The record stands as follows:

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The chronometer's error.

Personal

equation.

8hr. 55min. 54.I

The average of these five times, obtained by dividing their sum by 5, is 8hr. 55min. 33.58sec. That is the time, as nearly as the astronomer could estimate it, which the chronometer read when the star crossed his meridian. The book on the table states that the star really crossed the meridian at 8hr. 56min. 4.93sec.

The chronometer must therefore be in error; by subtracting the chronometer time from the time given in the book, we get the remainder 31.35sec. Shall we not say that the chronometer is 31.35 sec. slow? If the observer could estimate the time when the star crossed each spider-web accurately, and the instrument were perfectly adjusted in the meridian, one star would be sufficient. But the instrument has many errors, which must be taken into the reckoning, and the observer cannot do anything as accurately as he wishes. He therefore observes several stars, and applies the refinements of mathematical analysis to the problem in order to determine the errors of the instrument, and make allowance for them. From the observation of each star he obtains a value of the error of the chronometer; these he combines, taking their average as the final result.

When the utmost obtainable accuracy is desired, the "personal equation" of the observer must be taken into account. It takes time for men to think; the more

complicated the operation, the greater the time. In
the case of eye and ear observations, such as have
just been described, one impression reaches the brain.
through the eye, when the star crosses the wire.
Another impression comes from the chronometer, and
is transmitted by way of the ear. The brain is occupied
with the process of counting, but when the two impres-
sions arrive, it compares them, pronounces judgment,
and directs the hand to make a certain record.
man is especially trained he can do all this without
losing his count of the chronometer-beats.
even observe the times of transit across two or three
wires without removing his eye from the eyepiece, or
stopping to write anything down.

If a

He can

In the case of chronographic observations, which have been described in Chapter IX., and which are now generally used, the brain has much less to do. As before it receives an impression by way of the nerves of sight, and sends a mandate to the finger to touch the telegraph key. The mandate is obeyed, and the time is recorded almost instantaneously on the chronograph sheet. The personal equation for eye and ear observations is usually greater than for chronographic work, because of the greater complexity of the process.

Chronographic observations.

machine.

A machine has been invented for determining the personal equation of a time observer. The observer Personal looks through a little tube, resembling the eyepiece of a equation telescope, and sees an artificial star, which is driven by clockwork across a system of wires. The machine automatically records the time when the star crosses each wire; the astronomer presses his telegraph key, as usual, and thus records the time when he thinks that the star crosses each wire.

Such tests have demonstrated

that the average

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