Clock. This term designates generally any machine that uses a train of gear wheels to measure or show the various divisions of time. More specifically, it designates the ones located in the towers of churches and castles, in halls or on staircases, called long-case or table clocks.

In the beginning, they were called night dials , to distinguish them from sundials.

Although these timekeepers improved continuously after their invention, they were still very imperfect around the middle of the last century. However, once Huyghens invented, or improved, the use of a pendulum to replace the verge, in a short time they reached a degree of accuracy that could not have been hoped for without that fortunate discovery. See Horology.

Since the motion of a mechanical clock must be steady and last long enough to allow the measuring of time, the motion must first of all be produced, and then made uniform. There must be 1) a motive force, and 2) a series of parts that make the motion regular. Consequently, all clocks have a weight or a spring to produce the motion, and wheels and an escapement to modify it. This is the part of the clock the craftsman calls the movement . The other parts, which strike or repeat the hours, he calls the strike , repetition , etc. See Strike, Movement, Repetition, etc.

Description of large, or tower, clocks.

Since their invention and until about 1732, their overall design remained the same. Then the elder M. Le Roy invented the horizontal clock , which is undeniably better than the others. [1]

Our plates show a large horizontal clock as seen from above. The frame, which is rectangular, consists of bars AB, BC, CD, DA, held together by keys. The bars stand on edge, for greater rigidity. FE is another bar mounted in the same way, carrying the pivots for the striking and going movements. The rectangle EFCD contains the going movement: the great wheel is R, and the drum G is wound with the rope bearing the weight. The drum has a ratchet, q, which engages the spokes of the great wheel, allowing the drum to turn from G to X on the great wheel, but not from G to P. H is the second wheel, l is the escape wheel, and KF indicates the pallets attached to the pendulum, not shown here because the movement is seen from above. Thus, if weight P turns the drum, this will turn the great wheel, which will turn the second wheel, and so on until the escape wheel, which would turn as fast as determined by the weight, if the speed were not reduced and modified by the pendulum, which the escape wheel must cause to swing by engaging pallets K. So we see that weight P produces the motion, and the action of the pendulum on the escape wheel through pallets KK modifies it. The numbers [of teeth] for the wheels and pinions are: great wheel, 80; second wheel, pinion 10 and wheel 72; and escape wheel, pinion 8 and wheel 25. Since the great wheel is to make one revolution in an hour, it will readily be seen from these numbers that the pendulum will beat seconds. See Number , Vibration , Escapement , Pendulum , etc.

In this clock, as can be seen, the movement has three wheels. But the number of wheels is always a disadvantage, because they increase friction in the clock, and that increases irregularity. In consequence, when possible it is always advantageous to reduce the number of wheels, and in the present case it would be better to have only two wheels. This would have two advantages: friction would be reduced; and the pendulum would be longer, giving greater regularity. In the clock built for the Foreign Mission Seminary while my father observed, there are only two wheels, and a pendulum with a period of two seconds.

The winder consists of pinion N, which meshes with wheel O mounted on the drum; by means of crank 20, the weight is raised.

The strike train is lodged in the rectangle ADEF. The great wheel, the drum, and the winding pinion are 4, Z and Y; they are made like those of the going train, except that the great wheel carries nine pegs or pins, which engage the tail of the hammer. The second wheel is 12, the fly pinion is 21, and the fly is 18 and 19. The tail of the hammer is 6, 5, 9; part 9, as shown in the stem, extends between the pins. The first detent is r 9 a, for points c and b; this lock has a part, a, which must extend below part 3, 21 of the fly. The second detent is SUT, part of which, the counter, drops into the notches in the count wheel. Pin u on the shaft of the fly pinion is the stop for the strike; when first detent r 9 a is raised by the dial wheel, it raises detent ST by part S and releases it from pin u. At the same time the fly is stopped by part 21, 3, which encounters part a of the first detent, so the strike can begin only when that detent is no longer held up by the dial wheel; it drops and releases the fly pinion. The numbers are: great wheel, 81, and 9 for the winder. For the second wheel and the fly, the numbers are indefinite. See Strike. The count wheel has 90 teeth, and the pinion on the arbor of the great wheel has 9, so one revolution of the count wheel gives 90 strokes, the number a clock must strike in 12 hours, including the half-hours. See Strike.

Old tower clocks  [2] are essentially no different from this one as to the wheel trains, striking, fans, detents, etc., but there is a great difference in the frame and the position of the wheels. The frame consists of eleven parts, to wit: five uprights, four pillars, and two rectangles, upper and lower, rather similar to the clock we have just described. ³ Each rectangle is fitted and secured to the pillars in the same way as the rails BCAD with the bars CDAB. At the center of each is a crosspiece like EF, reinforcing the center upright. Two more uprights are located in the middle of the short sides of the rectangles and aligned with the first upright, to carry the wheels for the strike and time trains. The fourth upright is located at one of the sides of the rectangles, to carry the count wheel and the pinion that drives it. The fifth upright is opposite the one for the count wheel and carries the dial wheel or the bevel gear that drives it. This arrangement of posts in conventional tower clocks requires the time and strike trains to be arranged vertically or nearly so, hence the friction produced on the arbor of the great wheel by the weight is much greater than it might be otherwise. This disadvantage is eliminated in M. Le Roy's clock, which is all the more significant because the great wheel must make one revolution in an hour to release the strike. To grasp this fact, imagine a force at P driving the great wheel, and wheel H and its pinion stationary. Plainly, one can assume that pinion leaf e, against which the wheel tooth is bearing, is the fulcrum of the great wheel; since the wheel is pulled down by force P, its pivot is forced against the wall of its hole. To gauge the force, we may consider the distance between e and the arbor of the great wheel as a third-class lever, with its fulcrum, e, at one end, the weight or resistance at the other, and the effort, P, at the middle. Now we know that in a lever of this class the effort is always greater than the load; therefore, the pressure of the pivot against its hole, caused by the load, is less than the effort in proportion to the distance, de, between the drum and the fulcrum, and between the drum arbor and the same point.

Now let us assume the same effort at X rather than P, moving the wheel from G to X; the lever becomes second-class since the effort is at one end, the fulcrum at the other, and the weight or resistance is between the two. Now in a lever of this class the effort is always less than the load, so the pressure of the pivot against its hole, caused by the effort, will be greater than the effort itself, in proportion to the diameter of the drum plus the distance, de. Therefore, when the effort turning the wheel is between the arbor and the pinion, the pressure is always less than the effort; and when the effort is on the other side, and the pivot is between it and the fulcrum, the pressure is always greater. The friction varies with the pressure, therefore, etc.

So we see that whenever possible, the weight or effort that turns the great wheel must be between its arbor and the pinion with which it meshes.

1. These four paragraphs are italicized because they refer to a plate that is lacking. (translator's note).

2. This description of a traditional tower clock is italicized because it refers to a plate that is lacking (translator's note).