Sentence:  In a charge torpedo, a spring-pressed <WSD>percussor</WSD> recessed at one end, a guide tube in which said <WSD>percussor</WSD> works, a plummet adapted to lie in said recess and be held therein by the guide tube, a trigger adapted to engage said <WSD>percussor</WSD> and hold the same against the action of its spring, a pull cord secured to said plummet for shifting said <WSD>percussor</WSD> in the guide tube against the action of its spring, into position to be engaged by said trigger, ... 
Meaning of the word marked with <WSD> tags:  A part of a device that is intended to impart a striking action.