Sentence:  [M]otion is "transferred" and "modified"; by none of them "produced":—in all of them motion finds a "channel"; in none of them [...] a "source". [...] [W]hat are the several "sources of motion", and what are the corresponding "prime movers", or <WSD>"primum mobiles</WSD>"? 
Meaning of the word marked with <WSD> tags:  The prime mover or (effects follow).