Since 1530, when Gemma Frisius first proposed to ascertain the relative longitude of any place or ship at sea, by means of an horological machine for indicating the time of the first meridian, the subject had excited the attention of most of our philosophers, but unavailingly, as there was then no chronometrical instrument, upon which reliance might safely be placed. Huygens, in 1664, had contrived a time-piece actuated by a spring and regulated by a pendulum, but the pendulum was affected by the tossing of the ship, and by a change of temperature, as well as being subject, as was afterwards discovered, to a variation in weight depending on the parallel of latitude. The Academy of Sciences at Paris proposed, in 1720, a reward for the best paper in reply to the question:—'What is the most perfect method of preserving on the sea the equable motion of a pendulum?' The reward was given to a Dutchman named Massy, but his plan was not carried out. An English watch-maker named Henry Sully happened to be about this time in Paris directing a large manufactory of chronometers, and he presented the French Academy with a marine time-keeper of superior construction to the time-pieces of that period, and accompanied his gift by a memoir describing it. Whilst still engaged in the study of his art, Sully, who was a clever man, unfortunately died, and the opportunity of advance seemed to have passed away.
About this time Graham invented the Mercurial Compensation pendulum, which consisted of a glass or iron jar filled with quicksilver and fixed to the end of the pendulum rod, which, when heat lengthened the rod, expanded simultaneously the quicksilver, and made the centre of oscillation to continue at the same distance from the point of suspension. He afterwards conceived a notion, which John Harrison subsequently worked out, of making a compensation pendulum (or a pendulum that should in itself contain the power of equalizing its own action, whatever the change of temperature), forming it of various metals. In 1726 Harrison invented what is called the gridiron pendulum, composed of nine rods, five of steel and four of brass, which are so arranged that those which expand most are counteracted upon by those of less expansion. These two compensation pendulums, the gridiron and mercurial, are still in use, and with slight improvements are found to keep to time very accurately.
The period had now arrived for the making of marine time-keepers sufficiently accurate for nautical use, and styled chronometers because they are most accurate time-measurers. Their value to navigators, and the immense impetus which would by such instruments be given to the science of navigation, had long been foreseen, but there were many great difficulties in the way of obtaining a perfect chronometer. The sailor, before the invention of this instrument, could ascertain the latitude of his ship at sea, by observation of the fixed stars. Supposing these stars to have first appeared to him in the zenith, and at his next observation to be one, two, or three degrees south of the zenith, he would know that he had sailed just so many degrees north of the place in which he first observed them. It was not, however, so easy for him to compute longitudes, because the diurnal revolution of the earth causes each meridian to pass successively under the same stars. It was necessary to have an accurate time-keeper, and to set it carefully to the solar time of some port in the kingdom, whose longitude was well known. The time-piece might then be carried out in a vessel sailing abroad, and the computations made by means of it would prove most wonderfully exact and important. By simply observing the moment at which the sun reached his meridian, when of course it would be 12 o'clock at noon, solar time, and then noting the difference between the solar time thus ascertained and the time of the chronometer, the mariner would be able by calculating 15 degrees to one hour of time, or 15 geographical miles to one minute, to make out his longitude. For example, if the time-piece had been set to time at the meridian of Greenwich observatory, and if it be one o'clock by the time-piece when it is mid-day, or meridian by the sun, then the place in which the longitude is taken must be in long. 15 degrees east of the meridian of Greenwich, and if it be eleven o'clock by the chronometer when the sun attains his meridian, then the place must be in long. 15 degrees west of the meridian of Greenwich. It is not indispensably necessary, that every chronometer used for maritime purposes should keep time exactly with that of the Greenwich observatory, or of any other instrument of known excellence, provided always that its rate as seamen call it, or the daily loss or gain of the chronometer, is well ascertained, and so may be computed in the calculations to be made. 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.
It will be generally obvious of what immense and universal importance it was for men who 'go down to the sea in ships and do their business on the great waters' to be provided with a chronometer, and so be enabled to calculate with a great degree of nicety,—almost as a traveller by land learns his distances by milestones and finger-posts,—the precise position on the wide ocean of the vessel they are engaged in navigating. So impressed was the British Parliament with the value of such an invention, that as early as 1714, in the reign of Queen Anne, a reward of £10,000 was offered, for any method for determining the longitude within the accuracy of one degree; of £15,000 within the limit of 40 geographical miles; and of £20,000 within the limit of 30 geographical miles, or half a degree, provided such method should extend 80 miles from the coast. In 1736 John Harrison invented the first chronometer, for which, after having added many improvements, he received the gold medal of the Royal Society in 1749. He still continued to persevere in improvements in his instrument, and at last applied to be allowed to test its powers in such a voyage as might permit of proof of its value. After some time his application was granted, and his son, William Harrison, embarked at Portsmouth, Nov. 18, 1761, for Jamaica. After eighteen days sailing the vessel was computed to be 13° 50´ west of Portsmouth, when the distance calculated by the watch was 15° 19´. When the vessel arrived at Madeira, on the 9th of December, it was found that the reckoning was corrected by the time of the piece, about a degree and a half. From Madeira to Jamaica the reckoning was amended 3°; and at the several islands where the ship touched the known longitudes agreed very closely with those indicated by the chronometer. Upon having returned again to England after a very stormy voyage, the instrument underwent examination, and its entire error amounted to 1m. 53s. 5. Harrison, on this report being made, obtained from Parliament a reward of £5000. A second experiment was afterwards made in 1764, in March of which year Harrison left Portsmouth with his instrument on board the Tartar for Barbadoes. He had previously conveyed to the Lords of the Admiralty his statement of the rates at which his chronometer went, and the extent to which it was affected by change of temperature. On May 13th the vessel arrived at Barbadoes, and it was found that the amount of the daily deviations from mean time was only 43s. in excess. He returned to England after an entire voyage of 156 days, and found that, allowing the gain of one second per day as stated by him in his sealed 'rate,' the whole gain was only 54s. Harrison then was examined by a committee appointed for the purpose, and, having explained satisfactorily to them the principles of his instrument, he received another £5000. A trial was then made by another person with a chronometer made upon Harrison's plan, and this experiment also terminating favourably, the remaining parliamentary reward was paid over to Harrison, amounting in all to £20,000, a sum which was still further increased by gratuities from the Board of Longitude and the East India Company.
Harrison's improvements in time-measuring were of considerable importance, as any one may readily conceive, but he was sufficiently candid to acknowledge that the balance, balance-spring, and compensation curb, as then used, were not simultaneously affected by changes of temperature, that small pieces were more readily affected than large ones, and pieces in motion sooner than pieces at rest, whence he concluded that if the provision for heat and cold could properly be arranged in the balance itself, as in his gridiron pendulum-clocks, the time might be better kept.
Harrison's suggestion of a compensation balance in lieu of a compensating curb, incited Peter le Roy, a native of France, to the consideration of the question, and ultimately to the invention of a balance acted upon by mercury and alcohol. The compensation was effected by the balance itself, which, carrying the two thermometers, adjusted the mercury nearer or farther from the centre of the balance, according to the state of the atmosphere.
About this period there was considerable emulation exhibited, both here and on the continent, upon the subject of time-measuring. Sully had aided largely in the advancement of the art of watch-making in London and Paris. Berthoud, Julien, and Pierre le Roy made many ingenious propositions, and amongst others the invention of the detached escapement is attributed to the last-named.
In England we find the names of Arnold, Earnshaw, and Mudge associated about this date with the greatest improvements in chronometry, and as being those to whom prizes were at different times awarded by the Board of Longitude. In fact, few great inventions have since been made in the art, and our present high position as chronometer-makers is mainly due to the skill, energy, and perseverance then exhibited.
It would be superfluous to give any detailed description of the many valuable advantages derived from the science of horology, to which indeed all arts, sciences, trades, and callings are considerably indebted, and will probably be still more so in proportion to the increase of the use of steam-power and electricity. As by means of these recently-discovered powers mankind are enabled to compress into a day what would previously have required weeks and even months to accomplish, so must they regard with higher esteem, as these improvements are extended, the science by means of which they may divide and subdivide the precious minutes which are sufficient to perform so much. It will be worth while by way of illustration to point to the assistance given by horology to astronomical and nautical science. It is by means of carefully-made and exact chronometers that we calculate the distance and relations of the various heavenly bodies to ourselves and to one another. Having ascertained, by comparison, the rapidity of light and sound, and that the former travels at the rate of 192,000 miles per second, we discover that the light of the sun requires eight minutes to reach the earth, and thus compute the sun's actual distance from us. So also observing the number of seconds which elapse between the flash of lightning and the roll of thunder, or between the flash and report of a cannon, and remembering that in mild weather sound travels at the rate of 1123 feet, and in frosty weather 1080 feet in a second, we shall be able, on making allowances for the state of the atmosphere, to arrive at a tolerably correct conclusion as to distances. It is by means of a chronometer, though it be but a sand-glass, that the sailor uses his log-line at sea and finds the rate of his vessel's speed. His lead, enclosed in the log, or wood, is attached to the log-line, which has certain lengths called knots marked upon it for nautical miles, and according to the knots paid out in the half-minute of the sand-glass, so is the ship's rate of sailing, i. e., if ten knots are passed in half a minute the vessel's speed is at the rate of ten miles an hour.
It would be both impossible and unnecessary to describe the various experiments in which it is of great consequence to measure time into minute proportions, the number of these increases with advancing science; it will suffice if we have made the subject sufficiently interesting to the general reader to induce him to inquire further into the details. It is only by such investigations that he will be enabled to give anything like a proper answer to the question 'What is Time?'