CHRONOMETER. A watch of pecu liar construction, and great perfection of workmanship, used for determining geo graphical longitudes, or other purposes where time must be measured with ex treme accuracy. The chronometer differs from the ordinary watch in the principle of its escapement, which is so construct ed that the balance is entirely free from the wheels during the greater part of its vibration ; and also in having the balance compensated for variations of tempe rature. Marine chronometers generally beat half seconds, and are hung in gim bals, in boxes about six or eight inches square. The pocket chronometer does not differ in appearance from the ordi nary watch, excepting that it is generally a little larger. Chronometers are of im mense utility in navigation ; and ships going on distant voyages are usually fur nished with several, for the purpose of checking one another, and also to guard against the effects of accidental derange ment in any single one. The accuracy whith which some of the better sort of chronometers have been found toperform is truly astonishing ; the error in a two months' voyage not exceeding two or three seconds.
Chronometers, offered to the British Government to purchase, are placed in the Greenwich Observatory in the first or second week in January, and ranged on shelves round the chronometer room, and each is daily compared with the astro nomical clock, and its rate carefully noted. This is continued until the mid dle of July, during which time the tempe rature of the room is much varied. In the coldest weather the room is thrown open, so that it is as cold inside as out ; and in summer the change is all of 70° of difference. The chronometers arc also submitted day and night for about six weeks to 80 degrees heat, raised by fire. This is the usual trial.
Chronometers are more in use in Ameri can vessels than in those of any other na tion.
Mr. Loseby has introduced mercury into the chronometer to compensate for the loss of elasticity in the balance spring when subjected to heat. It acts equally by its fluidity and by its thermal expan sion, and has been favorably reported on after trial, by the Astronomer Royal of England.