Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/37

Rh the suspension nearer to the centre of gravity of the whole mass. But they are quite unfit for independent clock pendulums, having very little regulating power, or what we may call force of vibration. He applies magnets to both the bobs, so as to double the electrical force. Fig. 17 is the section across the plane of vibration. Fig. 18 shows the kind of reversed escapement, or "propelment," used with these short and slow pendulums. The pendulum here is returning from the extreme right, and has just deposited the right hand pallet BCD with its end D pressing on a tooth of the scape-wheel, but unable to turn it because another tooth is held by the stop G on the left pallet. As soon as the pendulum lifts that pallet the weight of the other pallet turns the wheel,until a tooth falls against the stop C. When the pendulum returns from the left the left pallet presses on a tooth at E but cannot turn the wheel because it is yet held by C, until that is released. In order to prevent the hands being driven back by wind where they are exposed to it, a click is added to the teeth. The wind cannot drive the hands forward by reason of the stops C, G.

Seeing that a clock at least the going part of it is a machine in which the only work to be done is the over coming of its own friction and the resistance of the air, it is evident, that when the friction and resistance are much increased, it may become necessary to resort to expedients for neutralizing their effects which are not required in a smaller machine with less friction. In a turret clock the friction is enormously increased by the great weight of all the parts; and the resistance of the wind, and sometimes snow, to the motion of the hands, further aggravates the difficulty of maintaining a constant force on the pendulum; and besides that, there is the exposure of the clock to the dirt and dust which are always found in towers, and of the oil to a temperature which nearly or quite freezes it all through the usual cold of winter. This last circumstance alone will generally make the arc of the pendulum at least half a degree more in summer than in winter; and in as much as the time is materially affected by the force which arrives at the pendulum, as well as the friction on the pallets when it does arrive there, it is evidently impossible for any turret clock of the ordinary construction, especially with large dials, to keep any constant rate through the various changes of temperature, weather, and dirt, to which it is exposed. Within the last twenty years all the best clockmakers have accordingly adopted the four-legged or three-legged gravity escapement for turret clocks above the smallest size; though inferior ones still persist in using the dead escapement, which is incapable of maintaining a constant rate under a variable state of friction, as has been shown before. When the Astronomer Royal in 1844 laid down the condition for the Westminster clock that it was not to vary more than a second a day, the London Company of Clockmakers pronounced it impossible, and the late Mr Vulliamy, who had been for many years the best maker of large clocks, refused to tender for it at those terms. The introduction of the gravity escapement enabled the largest and coarsest looking clocks with cast-iron wheels and pinions to go for long periods with a variation much nearer a second a week than a second a day. And the consequence was that the price for large clocks was reduced to about one-third of what it used to be for an article inferior in performance though more showy in appearance.

EB9 - Clocks - Fig. 19.png. 19.—Clock at Meanwood Church, Leeds.

Another great alteration, made by the French clockmakers before ours, was in the shape and construction of the frame. The old form of turret clock-frame was that of a large iron cage, of which some of the vertical bars take off, and are fitted with brass bushes for the pivots of the wheels to run in; and the wheels of each train, i.e., the striking, the going, and the quarter trains, have their pivots all in the vertical bar belonging to that part. Occasionally they advanced so far as to make the bushes movable, i.e., fixed with screws instead of rivetted in, so that one wheel may be taken out without the others. This cage generally stood upon a wooden stool on the floor of the clock room. The French clockmakers long ago saw the objections to this kind of arrangement, and adopted the plan of a horizontal frame or bed, cast all in one piece, and with such smaller frames or cocks set upon it as might be required for such of the wheels as could not be conveniently got on the same level. The accompanying sketch (fig. 19) of the clock of Meanwood church, near Leeds, one of the first on that plan, will sufficiently explain it. All the wheels of the going part, except the great wheel, are set in a separate frame called the movement frame, which is complete in itself, and light enough to take off and carry away entire, so that any cleaning or repairs required in the most delicate part of the work can be done in the clock factory, and the great wheel, barrel, and rope need never be disturbed at all. Even this movement frame is now dispensed with; but we will reserve the description of the still more simple kind of frame in which all the wheels lie on or under the great horizontal bed, until we have described train remontoires.

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