CHRONOMETER, among musicians, an instrument invented by Louie, a French musician, for the purpose of measuring time by means of a pendulum. The form of the instrument, as described by him, is that of an Ionic pilaster, and is thus described by Malcolm in his Treatise of Music, p. 407.—“The chronometer consists of a large ruler or board, six feet or 72 inches long, to be set on end; it is divided into its inches, and the numbers set so as to count upwards; and at every division there is a small round hole, through whose centre the line of division runs. At the top of this ruler, about an inch above the division 72, and perpendicular to the ruler, is inserted a small piece of wood, in the upper side of which there is a groove, hollowed along from the end that stands out to that which is fixed in the ruler, and near each end of it a hole is made: through these holes a pendulum cord is drawn, which runs in the groove: at that end of the cord which comes through the hole furthest from the ruler, the ball is hung; and at the other end there is a small wooden pin, which can be put in any of the holes of the ruler: when the pin is in the upmost hole at 72, then the pendulum from the top to the centre of the ball must be exactly 72 inches; and therefore, whatever hole of the ruler it is put in, the pendulum will be just so many inches as that figure at the hole denotes. The manner of using the machine is this: The composer lengthens or shortens his pendulum, till one vibration be equal to the designed length of his bar, and then the pin stands at a certain division, which marks the length of the pendulum; and this number being set with the clif at the beginning of the song, is a direction for others how to use the chronometer in measuring the time according to the composer's design: for with the number is set the note, crotchet, or minim, whose value he would have the vibration to be; which in brisk double time is best a minim or half bar, or even a whole bar, when that is but a minim; and in slow time a crotchet. In triple time, it would do well to be the third part or half, or fourth part of a bar; and in the simple triples that are allegro, let it be a whole bar. And if, in every time that is allegro, the vibration is applied to a whole or half bar, practice will teach us to subdivide it justly and equally. Observe, that, to make this machine of universal use, some canonical measure of the divisions must be agreed upon, that the figure may give a certain direction for the length of the pendulum.