Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/32

22  1em

1em 1em

1em

There are two kinds of striking work used in clocks. The older of them, which is still used in most foreign clocks, and in turret clocks in England also, will not allow the striking of any hour to be either omitted or repeated, without making the next hour strike wrong; whereas, in that which is used in all English house clocks, the number of blows to be struck depends merely on the position of a wheel attached to the going part ; and therefore the strik ing of any hour may be omitted or repeated without deranging the following ones. In turret clocks there is no occasion for the repeating movement; and for the purpose of describing the other, which is called the locking-plate movement, we may as well refer to fig. 22, which is the front view of a large clock, striking both hours and quarters on this plan. In the hour part (on the left), you observe a bent lever BAH, called the &quot; lifting-piece,&quot; of which the end H has just been left off by the snail on the hour-wheel 40 of the going part; and at the other end there are two stops on the back side of the lever, one behind, and rather below the other ; and against the upper one a pin in the end of a short lever 9 B, which is fixed to the arbor of the fly, is now resting, and thereby the train is stopped from running, and the clock from striking any more. The stops are shown on the quarter lifting-piece in the figure (27) of the Westminster clock. We omit the description of the action of the wheels, because it is evident enough. At D may be seen a piece projecting from the lever AB, and dropping into a notch in the wheel 78. That wheel is the locking-wheel or locking-plate ; and it has in reality notches such as D all round it, at distances 2, 3, up to 12, from any given point in the circumference, which may be considered as marked off into 78 spaces, that being the number of blows struck in 12 hours. These notches are shown in the locking-plate of the quarter part in fig. 22, but not in the hour part, foi want of size to show them distinctly. When the arm AB of the lifting-piece is raised by the snail depressing the other end H, a few minutes before the hour, the fly -pin slips past the first of the stops at B, but is stopped by the second and lower one, until the lever is dropped again exactly at the hour. Thus the pin can pass, and would go once round, allowing the train to go on a little; but before it has got once round, AB has been lifted again high enough to carry both stops out of the way of the fly-pin, by means of the cylinder with two slices taken off it, which is set on the arbor of the wheel 90, and on which, the end of the lifting-piece rests, with a small roller to diminish the friction. If the clock has only to strike one, the lifting-piece will then drop again, and the fly-pin will be caught by the first stop, having made (according to the numbers of the teeth given in fig. 22) 5 turns. But if it has to strike more, the locking- wheel comes into action. That wheel turns with the train, being either driven by pinion 20 on the arbor of the great wheel, or by a gathering pallet on the arbor of the second wheel, like G in fig. 15 ; and when once the lifting- piece is lifted out of a notch in the locking-plate, it cannot fall again until another notch has come under the bit D ; nd as the distance of the notches is proportioned to the 