Almost immediately after the invention of the attempts began to be made to introduce an escapement which would produce greater accuracy than the vertical escapement. Hooke, Huygens, Ilautefeuille, and Tompion introduced new principles, each of which has since been successfully applied, though they all, from imperfect execution, failed at the time. The first real improvement was made by George Graham, the inventor of the dead escapement in clocks. This is called the hori zontal escapement; it was introduced in the beginning of the last century, and it is still the escapement used in most foreign watches. - The impulse is given to a hollow cut in the cylindrical axis of the balance, by teeth of a peculiar form projecting from a horizontal crown-wheel. Other forms of escapement in high estimation are the lever escapement, originally invented by Berthoud, improved by Mudge; the duplex escape ment, the principle invented by Hooke, the construction perfected by Tvrer; and the detached escapement of Berthoud, improved by Arnold and Barnshaw. The last-men tioned is that which is employed in marine chronometers and in pocket-chronometers, as watches made in all respects like chronometers are called. The lever escapement is that which is used in most English watches. In it the scape-wheel and pallets are exactly the same as in the dead escapement in clocks. See HOROLOGY. The pallets are set on a lever which turns on their arbor; and there is a pin in a small disk on the verge or arbor of the balance, which works into a notch at the end of the lever. The pin and notch are so adjusted, that when a tooth of the scape-wheel has got free, the pin slips out of the notch, and the balance is detached from the lever during the remainder of its swing; whence the name detached lever escapement, originally applied to this arrange ment. On the balance returning, the pin again enters the notch, moving the lever just enough to send the tooth next in order to escape from the dead face of the pallet on to the impulse face; then the scape•wheel acts upon the lever and balance; the tooth escapes, and another drops upon the dead face of the pallet, the pin at the same time passing out of the notch in the other direction, leaving the balance again free. This arrangement is found to give great accuracy and steadiness of performance. To prevent the teeth from slipping away while the balance is free, the faces of the pallets are slightly undercut, and this makes them secure while at rest; moreover, there is a pin on the lever which moves through a notch on the balance disk, while the pin moves through the notch in the lever, which is so adjusted as to guard against the lever moving and the teeth escap ing, while the balance is free.
In watches, even more than in clocks, variations of temperature, unless provided for. produce variations in the rate of going, the increase or diminution of the tempera ture affecting to some extent the moment of inertia of the balance, and to a great extent the elastic force of the balance-spring. A rise in the temperature makes the balance expand, and therefore augments its moment of inertia; it adds to the length of the spring, and thereby diminishes its elasticity, the elastic force of a spring vary ing inversely as the length; and the time of vibration of the balance, which depeuds upon the moment of inertia directly, and upon the elastic force of the spring in versely, is increased—the watch, that is, goes more slowly—in consequence both of the increase of the inertia and of the diminution of the elastic force of the spring A fall hi the temperature is attended by opposite results, the watch going more rap: idly than before. A watch without a compensated' balance would vary very muck
more than a clock without a compensation pendulum, but that being usually carried in the waistcoat pocket, it is kept at a pretty uniform temperature. To invent a sat isfactory compensation was at one time the great problem for watch-makers. The compensation can be made in either of two ways—by an expedient for short ening the effective length of the balance-spring as the temperature rises, so as to increase the elastic force of the spring; or by an expedient for diminishing the moment of iner tia of the balance as the temperature rises, so as to correspond to the diminution of the force of the spring. The first method was that made use of by John Harrison (q.v.), who first succeeded in making a chronometer capable of measuring time accurately in different temperatures; but an adaptation of the other method, invented about eighty years ago by Earnsliaw, is that which is always em __ (Fig. ployeu now (Fig. tat' is the main bar of the balance, and t b, t' b' are two cempound bars, of which the outer part is of brass and the inner part of steel, carrying weights, b, b', which may be screwed on at different places. The brass bar expands more with heat, and contracts more with cold than the steel bar; therefore, as the temperature rises, the bars, with their weights, bend inward, and so the moment of inertia of the bal ance is diminished; as it falls, they bend outward, and the moment of inertia is increased; and of course the diminution or the increase must be made exactly to correspond to the diminution or increase in the force of the spring.
The chronometer ts just a large watch fitted with all the contrivances which experience has shown to be conducive to accurate time-keeping—e.g., the cylindrical balance-spring, the detached escapement, and the compensation-balance. As a watch which will keep time in one position will often not do so equally well in another, marine chronometers are always set horizontally in a box in gimbals (q.v.), an arrangement which keeps the chronometer horizontal, whatever the motion of the vessel.
The great importance of an accurate portable time-keeper at sea is for determining the longitude (q.v.). This use was first distinctly pointed out by sir Isaac Newton. A committee of the house of commons, of whom this philosopher formed one having been appointed on June 11, 1714, to consider the question of encouragement for the invention of means for finding the longitude, the result of their meetings was a memorial contain-. ing an explanation of the different means proper for ascertaining the longitude, and rec ommending encouragement for the construction of chronometers as the best means of ascertaining it. An act of parliament' was then passed, offering a reward for this purpose.
The first chronometer used at sea was invented by John Harrison. After many years of study it was completed in 173G. After several further trials and improvements, and two trial voyages to America, undertaken for the satisfaction of the commissioners, the last of which was completed on Sept. 18, 1764, the reward of £20,000 was finally awarded to Harrison.