Sentence:  For producing this "velocity of motion" we have a peculiar advantage, in having a double energy always at command, as half our crew are at rest always in the ordinary course of things, which, on particular emergencies, as in the instance of being chased by an enemy, may prove the salvation of the ship, cargo, and crew: not only may the whole muscular power of the crew be employed mechanically, in waorking the wheels of hydraulic engines ; but the different sets of muscles belonging to each man may be alternately employed and relieved by working them both <WSD>pedally</WSD> and manually, through the means of cranks, winces, and ropes. 
Meaning of the word marked with <WSD> tags:  By means of the foot.