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principle of ſecurity, which conſtitutes the peculiar excellence of this Lock.
The key, which is repreſented by the figure F, exhibits ſix different ſurfaces on its bit, againſt which the levers are progreſſively admitted, in the operation of opening the Lock; the irregularity of theſe ſurfaces deſcribes the diſtinct, and unequal degree of preſſure, which each lever requires to bring them to their proper bearings, for the purpoſe of putting the machine in motion. It hence appears, that unleſs the various heights of the ſurfaces, expreſſed on the bit of the key, are exactly proportioned to the ſeveral diſtances, to which the levers muſt be carried, to bring their notches into a direct line with each other, they muſt remain immoveable; and, as one ſtroke of a file, is ſufficient to cauſe ſuch diſproportion, as will prove an inſurmountable impediment to their motion; I may ſafely aſſert, that it is not in art to produce a key,
