A CHRONOMETER is neither more nor less than a very superior watch. Its motive power is a spring whose varying force (as it uncoils) is accurately compensated for by the form of the drum upon which the chain is wrapped. What is called the "escapement" (that is, the mechanism which prevents the watch from running down all at once) is somewhat different to that of an ordinary English lever watch. The seconds' hand only moves two steps for each second, and the balance-wheel receives an impetus only when swinging in one direction. The hair-spring is of the cylindrical form, and the balance-wheel itself is very carefully "compen­sated" for temperature. The result of all the succes­sive improvements in the construction of Chronometers, and of the extraordinary care taken in their manufacture, is that a good Chronometer will continue to go at a con­stant rate, even when subjected successively to two extreme temperatures (say 50° and 90°), and very nearly at the same rate for intermediate temperatures.

A Chronometer should be kept in a padded box in a part of the ship where it will be as free as possible from vibration, and should never be moved until taken ashore, and should be wound up regularly. The use of a Chrono­meter on board ship is mainly to keep Greenwich time from port to port; hence the navigator requires to know on leaving port the error of his Chronometer as compared with Greenwich Mean Time, and also the extent to which it is gaining or losing per day. This information is usually supplied by an Optician or Chronometer "rater," who has exceptional facilities for doing this important work. But the navigator takes every opportunity of checking the daily rate from time to time, for the rate may change from the moment the Chronometer is taken on board.

(b) SHIP TIME

For the purpose of discipline on shipboard and to divide the watch fairly, the crew is mustered in two divisions: the Starboard (right side, looking toward the head), and the Port (left). The day commences at noon, and is thus divided: Afternoon Watch, noon to 4 p.m.; First Dog Watch, 4 p.m. to 6 p.m.; Second Dog Watch, 6 p.m. to 8 p.m.; First Watch, 8 p.m. to midnight; Middle Watch, 12 a.m. to 4 a.m.; Morning Watch, 4 a.m. to 8 a.m.; Forenoon Watch, 8 a.m. to noon. This makes seven Watches, which enables the crew to keep them alternately, as the Watch which comes on duty at noon one day has the afternoon next day, and the men who have only four hours' rest one night have eight hours the next. This is the reason for having Dog Watches, which are made by dividing the hours between 4 p.m. and 8 p.m. into two Watches.

* The signals at the above places are made at 1 p.m. Greenwich mean time, with the exception of Dover, where the gun is fired at noon Greenwich mean time.

At most of these places no signal is made on Sundays or general holidays.

NOTE (Liverpool). — Chronometers tested gratis at Bidston Observatory.

Rule 1. — Divide the number of degrees, minutes and seconds by 15, and the quotient will be the time. If longitude is west of Greenwich, the result will be the time at Greenwich when it is noon at the place. Ex­ample: Longitude 74° 48' 15" W. What is the time ? Divide by 15 — 4 h., 59 m., 15 s., slower than Greenwich.

Rule 2. — To find difference in time between two places divide the difference in longitude by 15. Example: Paris, longitude 2° 20' E.; Philadelphia, longitude 75° 10' W. Difference in longitude 77° 30' divided by 15 — 5 h., 10 m., difference in time.

Rule 3. — To find difference in longitude (e.g. distance sailed) when difference in time is known, multiply the difference in time by 15.

(g) COMPARATIVE VELOCITIES