[Illustration: FIG. 210.--The hand-setting mechanism in action.]
In one form of this mechanism the push-piece is dispensed with, and the minute-wheel pinion is engaged by pulling the knob upwards.
[Illustration: FIG. 211.--The hour-hand train of a clock.]
The teeth of the mainspring drum gear with a cog on the minute-hand shaft, which also carries one of the cogs of the escapement train. The shaft is permitted by the escapement to revolve once an hour. Fig. 211 shows diagrammatically how this is managed. The hour-hand shaft A (solid black) can be moved round inside the cog B, driven by the mainspring drum. It carries a cog, C. This gears with a cog, D, having three times as many teeth. The cog E, united to D, drives cog F, having four times as many teeth as E. To F is attached the collar G of the hour-hand. F and G revolve outside the minute-hand shaft. On turning A, C turns D and E, E turns F and the hour-hand, which revolves 1/3 of 1/4 = 1/12 as fast as A.[41]
On these unfortunately necessary mechanisms a great deal of ingenuity has been expended. With the advance of luxury and the increased worship of wealth, it becomes more and more necessary to guard one's belongings against the less scrupulous members of society.
The simplest form of lock, such as is found in desks and very cheap articles, works on the principle shown in Fig. 212. The bolt is split at the rear, and the upper part bent upwards to form a spring. The under edge has two notches cut in it, separated by a curved excrescence. The key merely presses the bolt upwards against the spring, until the notch, engaging with the frame, moves it backwards or forwards until the spring drives the tail down into the other notch. This primitive device affords, of course, very little security. An advance is seen in the
The bolt now can move only in a horizontal direction. It has an opening cut in it with two notches (Figs. 213, 214). Behind the bolt lies the _tumbler_ T (indicated by the dotted line), pivoted at the angle on a pin. From the face of the tumbler a stud, S, projects through the hole in the bolt. This stud is forced into one or other of the notches by the spring, S^1, which presses on the tail of the tumbler.