Fig. 88.—Winding Arrangements.
To wind up the clock the barrel B, Fig. [88], is turned round by the key on the square; the pawl L fastened to the wheel G allows the barrel to be turned in one direction without turning the wheel. It is obvious, however that directly we begin to wind up, the pressure on the pawl tending to turn the wheel G is removed, and the clock stops—a very objectionable thing in astronomical and other clocks supposed to keep good time. The following is one of the devices for keeping the clock going during winding,—in this case everything is the same as before, with the exception of an additional rachet-wheel R2, Fig. [88], carrying the pawl L; this wheel is loose on the axis but attached to the wheel G through the spring S. The weight therefore acts on the pawl L, and tends to drive the wheel R2, which again presses round the wheel G by means of the spring S, and, as the whole moves round, the teeth of the wheel R2 pass the pawl K K fixed to some part of the clock-frame. When now we commence to wind, the pressure on the pawl L and wheel R2 is removed, and the spring S S, which is always kept bent by the action of the weight, endeavours to open; and since the wheel R2 is prevented from going backwards by the pawl K, the wheel G is continually urged onwards by the spring, and the clock kept going for the short period of winding.
II. The Pendulum.
The clock, as left by Henry de Wyck, was only an exceedingly irregular time-keeper, and some mechanical contrivance that should beat or mark correct intervals of time was urgently required. The contrivance for beating correct intervals of time—the pendulum—was thought of by Galileo, who showed that its oscillations were isochronous, although their lengths might vary within small limits. The pendulum then was just the very thing required, and Huyghens, in 1658, applied it to clocks.
In the next form of clock, therefore, we find the pendulum introduced as a regulator. There was a crown wheel like the one in the balance clock, only instead of being vertical it was horizontal. This wheel was allowed to go round and the weight was allowed to fall by means of alternating pallets; it was in fact like that shown in Fig. [86], with the balance weights and the rod carrying them removed, and instead thereof there was a rod, attached at right angles to the end of that carrying the pallets, and hanging downwards, which, by means of a fork at its lower end, swung a pendulum to an extent equal to the go of the balance first used. Thus the pendulum was adapted by Huyghens. We have here something extremely different from the rough arrangement in which the weight was controlled by the horizontal oscillating bar carrying the weights, for the balance would go faster or slower as the crown wheel pressed harder or softer against the pallets, and so, if the weight acted at all irregularly the clock would go badly. But with the pendulum the control of the weight over it is small, for the bob can be made of considerable weight, because it swings from its suspending spring without friction, and such a heavy weight at the end of a long rod is scarcely altered in its rate by variations of pressure on the pallets.
Galileo and Huyghens who followed him found that the oscillations of a simple pendulum are isochronous at all places where the force of gravity is equal, and that the time of oscillation depends on the length of the pendulum—the shorter the pendulum the shorter time of oscillation, and vice versâ. The time of oscillation varying as the square root of the length.
In 1658, then, the pendulum was applied to clocks, as the balance had been before that time. But Huyghens was not slow to perceive that the circular arc of a rigid pendulum would not be sufficiently accurate for an astronomical time-keeper, when used with a clock like that employed by Tycho Brahe and the Landgrave of Hesse for their astronomical observations. Huyghens next showed that with a clock of that kind, requiring a large swing of pendulum, the oscillations were not quite isochronous, but varied in time according as the arc increased or diminished. It was clear therefore that this simple form of pendulum would not do well for the large and varying arc required to be described, but that the theoretical requirements would be satisfied if the pendulum, instead of being suspended from a rigid rod, were suspended by a cord or spring or some elastic substance which would mould itself against two curved pieces of metal, C C, Fig. [89], attached one on either side of the suspending spring. In swinging, the spring would wrap, as it were, gradually round either curved surface, and so virtually alter the point of suspension, and with it, of course, the virtual length of the pendulum; so that the extreme point of the pendulum U, instead of describing a circular arc K B as before, would, by means of the portions of metal at the top, have a cycloidal motion D L, the pendulum becoming virtually shorter as the spring wrapped round the pieces of metal, so that it becomes isochronous for any length of swing. But it was very soon found that the theoretically perfect clock did not after all go as well as the clock it was to replace. And it would now be difficult to say what would have happened if a few years afterwards clocks had not been made much more simple and perfect by the introduction of an entirely new escapement which permitted a very small swing.
Fig. 89.—The Cycloidal Pendulum.
If we wish a clock to go perfectly well, we have only to consider a very few things—First, the weight should be as small as possible; secondly, within reason, the pendulum should be as solidly suspended and as heavy as possible; and, thirdly, the less connection there is between the pendulum which controls the clock, and the weight which drives the clock, the better.