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fig. 6. Interior of a back-spring warded lock.

The annexed cut, fig. 6, represents the interior of an ordinary back-spring lock, without tumblers. Such a lock may usually be known from a tumbler-lock by this simple circumstance, that it emits a smart snapping noise during the process of locking, occasioned by the pressure of the spring when the bolt is in a particular position. In the woodcut the bolt is represented half out, or half shot. At a a are two notches on the under side of the bolt connected by a curved part; b is the back spring, which becomes compressed by the passage of the curve through a limited aperture in the rim c c of the lock. When the bolt is wholly withdrawn, one of the notches a rests upon the rim c c; and the force with which the notch falls into this position, urged by the spring b, gives rise to the snapping or clicking noise. When the bolt is wholly shot, the other notch rests in like manner upon the edge of the aperture in the rim.

It must be obvious at a glance, that this back-spring lock is objectionable on the score of security, on account of the facility with which the bolt may be forced back by any pressure applied to its end, a pressure which may often easily be brought to bear. At the centre of the lock is seen the end of the key acting on a notch in the bolt, and surrounded by wards.