CHRONOMETER, an instrument or machine for measuring time. The word is more particularly used by workmen and navigators to denote a watch, or portable machine, in which, by the na ture•of the escapement and the com pensations for heat and cold, mean time is or ought to be kept with sufficient accuracy to determine the longitude at sea.
The relation between time and longi tude will be fully explained hereafter: it will therefore be sufficient in this place to remind the reader, that the rotation of the earth upon its axis brings the seve ral places upon its surface, in succession, opposite. the sun, causing day and night ; so that the absolute instant of noon, or of any other determined apparent time of the day, each place must be earlier, at a place which lies to the eastward of another, with which that place may be compared. From this general fact it follows, that allowing 24 hours for the whole rotation of the earth, and pro portionally for every smaller part of the rotation, we may determine tprovided the, apparent time at two places be known) what is the difference of longi tude between them. Thus, if a chrono meter set to the time at Greenwich were to be carried to Petersburgh, in Russia, it would indicate time two hours later than the clocks at Greenwich ; that is to say, it would shew when it was noon at Greenwich, instead of when at Peters burgh. The obvious conclusion would be, that the sun arrives at the meridian of Petersburgh earlier, and consequently that this town lies more easterly than Greenwich ; and as two hours are in pro portion to 24 hours, so is 360°, the earth's circumference, to 30°, the longitude of St. Petersburg, reckoned from Green wich. Upon the same principle it is, that the clocks in a large town ought not to indicate thesame time. Thus the clocks at St. Paul's, St. Clement's, St. Martin's, and St. George's, Hanover Square, in London, ought to strike each four seconds after the other ; and this difference, it may be added, would nearly vanish, if heard from any of the westerly stations, on account of the time employed for the passage of sound; and for the same rea son it would be nearly doubled in the op posite direction.
From the intimate relation which sub sists between the construction of watches and clocks, the similitude of the escape ments, and the common principles upon which the compensations for heat and cold are effected in each, we shall ex plain the principles of each under the general article HosoLoox ; and at pre sent we shall only give an account of the nature of the expedients adopted to produce superior accuracy in these port able machines.
The train of wheels, which constitutes so large a part of every time-piece, must necessarily transmit the force of the first mover with periodical irregularities, arising from oblique actions of their teeth upon each other ; and these irregulari ties will be subject to other variations, arising from the greater or less degree of fluidity in the oil applied to the pivots and elsewhere. The first mover also in a portable machine being a spring will be more rigid, and consequently act with greater power when cold than when hot. The balance, or vibrating measurer of the time, is a wheel, or equivalent piece, fixed on an axis, upon which it could freely turn ; but this liberty is restrained by a fine spring, called the pendulum spring, which is fastened to the axis, and after taking several turns round without touching it, the other end of the spring is fixed to the frame of the ma chine. By this contrivance the balance will, if not prevented, come to rest in one particular position ; and if at any time disturbed, it will only vibrate each way from the line of quiescence, performing larger or smaller arcs, according to the disturbing force. This force in a watch or time-keeper is communicated from the train ; most commonly during the time of each vibration ; and the machinery oz.