in general, denotes any instrument or machine used in measuring time; such are dials, clocks, watches, &c. See Dial, &c.

The term chronometer, however, is generally used in a more limited sense, for a kind of clock so contrived as to measure a small portion of time with great exactness, even to the sixteenth part of a second; of such a one there is a description in De la Guerre's experimental philosophy, invented by the late ingenious Mr George Graham; which must be allowed to be of great use for measuring small portions of time in astronomical observations, the time of the fall of bodies, the velocity of running waters, &c. But long spaces of time cannot be measured by it with sufficient exactness, unless its pendulum be made to vibrate in a cycloid; because otherwise it is liable to err considerably, as all clocks are which have short pendulums that swing in large arches of a circle.

There have been several machines contrived for measuring time, under the name of chronometers, upon principles very different from those on which clocks and watches are constructed.

Plate CXXXVII, Vol. V, fig. 1 represents an air-chronometer, which is constructed in the following manner: Provide a glass tube of about an inch in diameter, and three or four feet long; the diameter of the inside of this tube must be precisely equal in every part: at the bottom must be a small hole, closely covered with a valve. In the tube place a piston, E, fig. 2, which is made to fit it exactly, and must be oiled, that it may move in the tube with the greatest freedom; in this piston there is a cock that shuts quite close; and from the top of it there goes a cord F, which passes through the handle G. The cock of the piston being closed, it is to be let down to the bottom of the tube, and being then drawn up to the top, the air will then rush in by the valve at the bottom of the tube, and support the piston. You are then to turn the cock, so as to make a very small vent; and the air passing slowly through that vent, the piston will gradually descend and show the hour, either by lines cut in the tube with a diamond, or marked with paint, or by small slips of paper painted on the glass. If this chronometer should go too fast or too slow, it may be easily regulated by altering the position of the cock in the piston, as it is on that the whole depends.

If, instead of marking the tube, you would have the time shown by a dial, it may be easily effected by placing an axis, to which the hand of the dial is fixed directly over the tube, and winding the string to which the piston is joined round that axis; for then as the piston descends, the axis will gradually turn the hand, and show the hour; but it must be observed, that as the descent of the piston is not constantly regular, on account of the decrease of resistance from the quantity of the subjacent air as the piston descends, the axis therefore must not be a regular cylinder, but conical like the fusee of a watch, as in fig. 3, by which means the motion of the hand of the dial will be constant and regular.

Fig. 4. represents a lamp-chronometer. It consists of a chamber lamp A, which is a cylindrical vessel about three inches high, and one inch diameter, placed in the stand B. The inside of this vessel must be everywhere exactly of the same diameter. To the stand B is fixed the handle C, which supports the frame DEFG, about 12 inches high, and four wide. This frame is to be covered with oiled paper, and divided into twelve equal parts by horizontal lines; at the end of which are wrote the numbers for the hours, from 1 to 12, and between the horizontal lines are diagonals that are divided into halves, quarters, &c. On the handle B, and close to the glass, is fixed the style or gnomon H. Now, as the distance of the style from the flame of the lamp is only half an inch, if the distance of the frame from the style is only five inches, then, while the float that contains the light descends by the decrease of the oil one inch, the shadow of the style on the frame will ascend 12 inches, that is, its whole length, and show by its progression the regular increase of the hours, with their several divisions. It is absolutely necessary, however, that the oil used in this lamp be always of the same sort and quite pure, and that the wick also be constantly of the same size and substance, as it is on these circumstances, and the uniform figure of the vessel, that the regular progress of the shadow depends.

among musicians, an instrument invented by Loulie, 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, five feet or 72 inches long, to be set on end; it is divided into 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," Chronometer 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 uppermost 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 cliff 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 duple 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.