Chronometer

“ Harrison's experience with the compensation pendulum of his clock now proved of service to him. He had proceeded to introduce a similar expedient in his proposed chronometer. As is well known to those who are acquainted with the nature of springs moved by balances, the stronger those springs are, the quicker the vibrations of the balances are performed, and vice versa; hence it follows that those springs, when braced by cold, or when relaxed by heat, must of necessity cause the timekeeper to go either faster or slower, unless some method could be found to remedy the inconvenience. ”

“ Try to imagine “All larboard bowlines on deck!” being shouted down into the forecastle of a present day ship. Yet that was the call used on the Pilgrim to fetch Dana and the rest of his watch on deck.
The chronometer, which is merely the least imperfect time-piece man has devised, makes possible the surest and easiest method by far of ascertaining longitude. Yet the Pilgrim sailed in a day when the chronometer was just coming into general use. So little was it depended upon that the Pilgrim carried only one, and that one, going wrong at the outset, was never used again. ”

“ The chronometer mainly differs from the ordinary watch in being formed of a mixture of metals, which preserve so nice a chemical balance, that those changes of temperature which quicken or retard the movements of common time-pieces fail to affect it. Now, let us suppose that the friend and adviser of the sailor had said to him,—using a common metonymy,—there are no chronometers anywhere constructed that so completely neutralize the temperature as the ones I recommend to you ”

“ At the Royal Observatory, Greenwich, there are frequently above 100 chronometers being rated, and there have been as many as 170 at one time. They are rated daily by two observers, the process being as follows. At a certain time every day two assistants in charge repair to the chronometer-room, where is a time-piece set to true time; one winds up each with its own key, and the second follows after some little time and verifies the fact that each is wound. One assistant then looks at each watch in succession, counting the beats of the clock whilst he compares the chronometer by the eye ”

“ By means of these instruments, the error of the chronometer is found at each station with great accuracy, and, the times shown by the faces of the chronometers being compared by telegraph, the difference of time and corresponding difference of longitude are readily deduced.
The time occupied by an electric impulse to traverse the wire from one station to another, and act upon the telegraph-instruments, though generally very small, is too great to be neglected, but is easily ascertained and allowed for. ”

“ The indispensable requisite of a chronometer, however, is that the daily loss or gain shall not vary materially from itself at different periods, or under the changes of temperature of different climates, and these qualities being found in an instrument of any shape or make, constitute a marine chronometer. ”

“ In the swift transportation of manufactured goods and agricultural products, it has become a most efficient incentive to human industry
The perfection of ocean steam-navigation was greatly promoted by the invention of the chronometer, which rendered it possible to find with accuracy the place of a ship at sea. The great drawback on the advancement of science in the Alexandrian School was the want of an instrument for the measurement of time, and one for the measurement of temperature—the chronometer and the thermometer; indeed, the invention of the latter is essential to that of the former. ”

“ A spring-driven chronometer is set approximately to Greenwich mean time (GMT) and is not reset until the instrument is overhauled and cleaned, usually at three-year intervals. The difference between GMT and chronometer time (C) is carefully determined and applied as a correction to all chronometer readings. This difference, called chronometer error (CE) , is fast (F) if chronometer time is later than GMT, and slow (S) if earlier. The amount by which chronometer error changes in 1 day is called chronometer rate. An erratic rate indicates a defective instrument requiring repair. ”

“ While we may well marvel at the precision of the chronometer, it is equally marvelous, if not more so, that we may equip ourselves for a few dollars with [68] a timepiece which is so wonderfully accurate as to vary little more than a second per day. If one took the pains to regulate his watch carefully, any of the better makes could be adjusted to such accuracy.
There is nothing very mysterious about the mechanism of a watch. It consists merely of a train of gears which slow down the motions of the mainspring to a convenient speed ”

“ If with the naked eye or with a telescope you watch a clock moving away from you, you will find that its minute hand takes a longer time to cover its five-minute intervals than does the chronometer in your hand, and if the clock travelled with the velocity of light you would forever see the minute hand at precisely the same point. That which is true of the clock is, of course, also true of all time intervals which it measures, so that if you moved away from the earth with the velocity of light everything on it would appear as still as on a painted canvas. ”

“ Every one knows the beautiful machinery of a timepiece, and the perfect tools required to produce such a machine. Some of these tools Harrison procured in London, but the greater number he provided for himself; and many entirely new adaptations were required for his chronometer. As wood could no longer be exclusively employed, as in his first clock, he had to teach himself to work accurately and minutely in brass and other metals. ”

“ A consideration of this escapement will show that it is only the application of the detached chronometer escapement to a clock.
Even detached escapements, however, are not perfect. In order that an escapement should be perfect, the impulse given to the pendulum should be always exactly the same. It may be asked why, if the time of oscillation of the pendulum be independent of the amplitude of the arc of vibration, and hence of the impulse, it is necessary that the impulse should be uniform. ”

“ The construction is perfectly successful; the chronometer remains in coincidence with the Transit Clock through any length of time, with a small constant error as is required by mechanical theory."—"The printed volume of Observations for 1864 has two Appendixes; one containing the calculations of the value of the Moon's Semi-diameter deduced from 295 Occultations observed at Cambridge and Greenwich from 1832 to 1860, and shewing that the Occultation Semi-diameter is less than the Telescopic Semi-diameter by 2" ”

“ Every ship carries a marine chronometer, which is a time-keeper of the most careful construction, whose accuracy has been tested with the utmost precision—which is plainly a matter of the utmost importance to a vessel when it is remembered that an error of four seconds of time represents about one and one-tenth statute miles, enough to wreck a ship on a lee shore when the captain, following the chronometer, supposed himself at a safe distance from the shoal or shore. ”

“ Six days later — that is, on the 30th June — the Board of Longitude met, when Harrison was present, and produced the chronometer with which he had made the voyage to Lisbon and back. The minute states: "Mr. John Harrison produced a new invented machine, in the nature of clockwork, whereby he proposes to keep time at sea with more exactness than by any other instrument or method hitherto contrived, in order to the discovery of the longitude at sea ”

“ The above method of determining the geographical meridian has the serious objection that it is necessary to know the error of the chronometer with very considerable accuracy, a matter of some difficulty when observing at any distance from a fixed observatory. If, however, a theodolite, fitted with a telescope which can rotate about a horizontal axis and having an altitude circle, is employed, so that when observing a transit the altitude of the sun or star can be read off, then the time need only be known to within a minute or so. ”

“ Ultimately, however, the accuracy of the watch, and its indispensableness in the perfected form of the chronometer, having been fully demonstrated, the method of lunars became practically obsolete and the mariner was able to determine his longitude with the aid of sextant, chronometer, and Nautical Almanac, by means of direct observation of the altitude of the sun by day and of sundry fixed stars by night, a much simpler calculation sufficing than was required by the older method. ”

“ Both the first success of the chronometer and the later efforts toward improving it had a great influence upon the next few generations of watchmakers; the final improvements were made in the days of the American Revolution. It was at this latter period that a man named Thomas Mudge worked out the kind of escapement that is still used in our watches. ”

“ It depends wholly on the measurement of time as the stars to be compared drift across the ring while the telescope is fixed, and while a clock or chronometer operating a sounder is a desirable adjunct one can do pretty well with a couple of stop watches since only differential times are required. ”

“ We would beg to say here there is no escape wheel made which requires the same extreme accuracy as the chronometer, as the tooth spaces and the equal radial extent of each tooth should be only limited by our powers toward perfection. It is usual to give the detent a locking of about two degrees; that is, it requires about two degrees to open it, counting the center of fluxion of the detent spring E and five degrees of balance arc. ”

“ The observer watches the luminous point, and as soon as he sees it upon the line presses a button which stops a second chronometer. The difference between the times indicated by the two chronometers gives the personal equation of the observer for the transits of stars, recorded by pressing a button. Its amount will be very small. When the time is taken by glancing at a clock and then noting, the equation of almost all observers is so large as to demand a correction. ”

“ But since then many improvements have been made in it, so that makers are now enabled to produce a pocket watch, with the short angle lever escapement, which marks time at a steady rate of within four or five seconds weekly,—a rate which approaches so near to the time-keeping of a pocket chronometer, that unless the minutest exactness for some specific purpose is required, the last-named watch is all that can be wished for. ”

“ The chronometer, for finding longitudes, has taken the place of lunar observations, and the story of the struggle to invent a time-keeping mechanism of requisite accuracy for use at sea is one of the romances of science. Watches were unknown until 1530, but before the end of that century efforts had been made to ascertain the difference in time between two places by means of two of these crude timepieces which, however, were too unreliable to be of any practical service to navigation. ”

“ If some one should present you with a most delicately adjusted chronometer,—one which would not vary a second in a year—do you think it would pay you to trifle with it, to open the case in the dust, to leave it out in the rain overnight, or to put in a drop of glue or a chemical which would ruin the delicacy of its adjustment so that it would no longer keep good time? ”

“ It is true, chronometers are intended to keep the most accurate time, and usually they do; this of mine, in particular, would not lose ten seconds in a twelvemonth, did I not carry it on my person.”
“No, no, it does not seem to lose any, Harry; it only gains,” cried Rose, laughing.
Mulford was now satisfied, notwithstanding all that had passed on a previous occasion, that the laughing, bright-eyed, and quick-witted girl at his elbow, knew no more of the uses of a chronometer than her unusually dull and ignorant aunt ”

“ Bearings. with sextant and artificial horizon are necessary, unless the precise errors of the chronometers are known, when the time can be obtained by carrying a pocket chronometer to the station. The sun should be near the prime vertical and at a low altitude; the theodolite must be very carefully levelled, especially in the position with the telescope pointing towards the sun. To eliminate instrumental errors the observations should be repeated with the vernier set at intervals equidistant along the arc, and a.m. ”

“ Daily observations of the fall of this ball will give the daily gain or loss of the chronometers; that is, their rates.
A capital advantage will be that such corrections and rates can be determined without removing the chronometer from the ship, so that a fertile source of disturbance which accompanies the carriage of the chronometer to and from the vessel is thus, in future, avoidable. To the citizens of New York and Brooklyn the ball is widely visible. ”

“ I have never found another chronometer sensibly affected by magnetism.—In November and December I tried my new double-image micrometer.—Between May 16th and Oct. 13th a fireproof room was constructed in the southern part of the quadrant room; and in November a small shed was erected over the entrance to the North Terrace.—The position of the free Meridional Magnet (now mounted in the Magnetic Observatory) was observed at every 5 m. through 24 hours on Feb. 22nd and 23rd, May 24th and 25th, Aug. 30th and 31st, and Nov. 29th and 30th. ”