The general arrangement of the clock train is shown in Fig. 87, where W is the weight, hung by a cord passing over the barrel B, on the axis of wheel G. The teeth of the wheel G gear into the pinion P_{1}, which again is carried on the axis of the wheel C, and so on up the last wheel—the escape-wheel, which generally is cut to thirty teeth, so that it goes round once a minute and carries a second-hand. The pinion P_{1} is so arranged by the number of teeth between it and the escape-wheel that it goes round once an hour or to sixty turns of the escape-wheel. [Illustration: FIG. 87.—The Clock Train. ] [Illustration: 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 R_{2}, 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 R_{2}, which again presses round the wheel G by means of the spring S, and, as the whole moves round, the teeth of the wheel R_{2} 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 R_{2} 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 R_{2} 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.