CHRONOMETER. Such time-keepers as are used for determining the longitude at sea, or for other purposes where great accuracy is required, combined with portability, are called by this name. Some are made for the pocket,. resemble ordinary watches in appearance, and, like them, are made to go thirty hours with once winding up. Marine chronometers are usually larger, with dial•plates three or four inches in diameter, and made to go from two to eight, or, by some recent improvements, even as many as thirty-two days with once winding up. They indicate hours (from I. to XXIV., to avoid the possibility of an error of twelve hours), minutes, and seconds, often on sepa rate circles ; and they are furnished with a small hand to indicate the days which have elapsed since the winding up. The mechanism is enclosed in a brass box, mounted on gim bals, so as to preserve one uniform position, and inclosed in a mahogany case.
The greater part of the mechanism of chronometer resembles that of a common watch, but is constructed with every possible provision to ensure accuracy. They differ from the common watch chiefly in the detached escapement and the compensation balance. The former is a contrivance by which the balance, the vibrations of which constitute the actual measure of time, is, during the greater part of its movement, entirely detached from, and ' consequently unaffected by, the train of wheel work from which its impulse is derived; the wheels, in fact, standing still through the greater part of each vibration. The compen sation-balance is a beautiful contrivance for counteracting the effect of changes of tempe rature, which, by causing an ordinary balance to expand and contract, occasion a variation in the extent of its vibration, and consequently in the rate of going of the time-piece of which it forms the essential feature. This correc tion, in the ordinary compensation-balance, is effected by forming the rim of the balance of two semicircular slips of metal, fixed at one end only, and each consisting of a very narrow riband of steel joined to an outer rim of brass. Each of these semicircular slips, consisting thus of two metals differently affected by heat, is capable of altering its shape with every change in the temperature to which the chro nometer is exposed, in, such a way as to keep the vibrations of the balance always the same. In the first marine chronometers, constructed by Harrison, this object was attained by apply ing such a compensating apparatus to the 0 balance-spring. Arnold, following, probably unconsciously, in the track of the great French watch-maker, Le toy, applied it to the balance itself. Many other improvements have been made in the chronometer, render ing it almost as perfect BA any thing from the hand of man can be expected to bo ; and of late years very ingenious contrivances have been devised by Messrs. Dent, Eiffe, and Mo
lyneux, for correcting certain irregularities of action in the ordinary compensation-balance and producing a balance surpassing it in accu racy of movement as it surpasses the balance of a common watch.
The wonderful accuracy of modern ehrono meters was made the means in 1830 of deter mining the longitude of New York, as corn. pared with Greenwich. The first experiment was made on the occasion of the first trip across the Atlantio by the British Queen. The longitude determined by that mode differed 2.63 seconds from that determined by the usual astronomical means. Mr. Dent was dissatisfied with a discrepancy—marvellously small to the minds of others—but large in the estimation of one accustomed to the per formances of chronometers ; and he deter mined to made a second attempt. He placed four chronometers under the care of Captain Roberts, the rates of tyhich had been pre viously ascertained. The chronometers made a voyage out and home in the British Queen, showing, of course, areenteich time all the while ; and on being compared in America with other chronometers which marked New York time, the difference of longitude was measured by this difference of chronometer time. So astounding was the accuracy of those instruments, that while the difference of longitude between the observatory at Green wich and the City Hall at New York is found by the best astronomical methods to be 4 h. 56 m. 0-72s., it was measured by the chronometers to be 4 11.56 in. 0.24 s.,—a differ ence of less than half a second! It was found that the chronometers were able to withstand the shaking of the vessel, and all other dis turbances incidental to a steam-ship, without perceptibly marring their action.
Mr. Dent, shortly afterwards, determined by chronometric means the difference of Ion gitude between Greenwich and Devonport As ascertained by the Trigonometrical Survey the difference of longitude, between the Ob servatory at Greenwich and the flag-staff Pt Mount Wise, near Davenport, is 16 na. 38'1 s. while the difference given by a mean of foul chronometers was 16 m, 39.8 s.—a difference 01 less than two seconds.
During the last five years, there have been 219 Admiralty chronometers at Greenwich Observatory. It is curious that in these sue, cessive years, 1845-6-7-8-0, five different makers stood successively first on the list, in respect to the accuracy of the chronometers made by them; showing how closely the spirit of honourable scientific competition has brought them on a level one with another. Of the whole number 79 were purchased for the public use ; the highest price paid was 681. Ps.