The =time lock= is illustrated in Fig. 54, and is a locking device actuated by clockwork. This type of lock is used in connection with the heavy boltwork of a safe door, to prevent it from unlocking except during certain hours determined beforehand. This lock is now recognized as an essential part of the equipment of all first-class, burglar-proof vaults and safes. In its standard form, the time lock has three chronometer movements of the finest construction, each of which is competent to actuate the lock, so that, in this way, almost absolute reliability is insured. [Illustration: FIG. 55] A =dial=, or =combination, lock= is shown in Fig. 55. The combination commonly called the _dial lock_ has completely superseded the key lock for use on safe and vault doors. Essentially, the dial lock consists of a bolting mechanism guarded by a set of changeable tumblers, or wheels. These tumblers are actuated by a spindle passing through the door, this spindle being provided on the outer end with a graduated dial. By rotating the dial in a certain manner, the dial can be set and the lock operated. Dial, or combination, locks are made in two grades, or varieties, designed respectively for burglar-proof and for fireproof safes. The lock intended for burglar-proof purposes is heavy and made to resist violence, while the other locks are smaller, simpler, and cheaper, intended only to secure the door against ordinary intrusion. Both of these locks should be absolutely “non-pickable.” In connection with the time lock, an automatic bolt-operating device is now used on burglar-proof safes. The =bolt-motor= is a mechanism containing heavy springs and is attached to the inside of a safe door. These springs are set, or braced, while the door is closed, and when released by the action of the time lock are capable of automatically retracting the heavy boltwork of the door. This construction obviates the necessity of any spindle through the door, and leaves the surface absolutely unbroken, without any communication between the interior and exterior. [Illustration: FIG. 56]