Greek, and meaning a time-measurer,--is chiefly applied to marine time-pieces and to watches which have been carefully made with chronometer or detached escapements and compensating balances serving to equalize the effects of heat and cold. MARINE CHRONOMETERS are the chief instruments for discovering the longitude at sea, and are therefore subjected to special tests at Greenwich observatory and elsewhere before being sent on board ship. They have dials of three or four inches in diameter, hour, minute, and second hands, besides a hand to indicate the day upon which the instrument was last wound up,--and they are made to go from two to eight days. Being well mounted on gimbals inside of an air and water-tight brass case they do not toss about with the motion of the ship but always preserve their equilibrium. For extra protection they are generally kept enclosed in a mahogany case. Chronometers have for their motive power, like watches and spring-clocks, a mainspring acting on the fusee by the chain,--as the chain winds upon the fusee the force of the spring is so equalized that it is exactly the same whatever the position of the chain. When marine chronometers are sent to the Greenwich observatory they are subjected, under the directions of the Astronomer Royal, to extreme degrees of heat and cold, and up to the year 1835 prizes were awarded to those makers whose instruments best stood these tests; but such prizes are no longer given. It has even been found that chronometers which are most capable of withstanding extremes of temperature are not the most perfect in medium climates, and this discovery brought about new endeavours and a new suggestion known as the Auxiliary or Secondary Compensation. MARINE TIME-PIECES FOR SHIPS AND YACHTS. These instruments possess the character rather of clocks than of chronometers, inasmuch as they are designed to hang against a bulk-head, and they would not appear unsuitable to house purposes. They are portable and useful clocks, and having a lever escapement with compensated balance, the motion of the vessel does not affect them. Some yacht time-pieces are constructed so as to chime the quarters or tunes, and to strike the ships' bells as well as the hours. They are also sometimes placed in very handsome cases of bronze or ormolu, decorated with special designs to illustrate the name of the ship or yacht to which they belong. Their movements are not as accurately adjusted as those of Marine Chronometers, but they, nevertheless, are made to keep time excellently. KEYLESS WATCHES.