upwards into the fixed cylinder F (see Fig. 221), or some of the pins in F will sink into G. It is then impossible to turn the key.
There are other well-known locks, such as those invented by Bramah and Hobbs. But as these do not lend themselves readily to illustration no detailed account can be given. We might, however, notice the time lock, which is set to a certain hour, and can be opened by the right key or a number of keys in combination only when that hour is reached. Another very interesting device is the automatic combination lock. This may have twenty or more keys, any one of which can lock it; but the same one must be used to unlock it, as the key automatically sets the mechanism in favour of itself. With such a lock it would be possible to have a different