Sentence:  The position of the <WSD>eccentrics</WSD> which is necessary to make the pistons drive the engine forward must be directly the reverse of that which would cause them to drive the engine backwards. To be able, therefore, to reverse the motion of the engine, it would only be necessary to be able to reverse the position of the <WSD>eccentrics</WSD>, which may be accomplished by either of two expedients. 
Meaning of the word marked with <WSD> tags:  A disk or wheel with its axis off centre, giving a reciprocating motion.