NATURE OF THINGS. 447 



part in the investigation, if they but pronounce such 

 motion to be violent, and contradistinguish it from natural. 

 And no doubt it is the system of Aristotle and his school, 

 to instruct men what to say, not what to think ; to teach a 

 man by what devices of affirming or denying, he may get 

 clear of a disputant in argument, not how by force of 

 thought, he may get clear of a difficulty in the conviction 

 of his own mind. Others rather more attentive, laying hold 

 of the position that two bodies cannot exist in one place, 

 say that it follows as a consequence that the stronger body 

 propel, and the weaker be dislodged : that this dislodging 

 or flight, if a less force is used, continues no longer than 

 the duration of the original impulse, as in protrusion ; but if 

 a greater, the impulsion continues for a time, even after the 

 removal of the impelling body, till it gradually slackens as 

 in throwing. And here again, according to another inve 

 terate habit of the same school, they catch only at the com 

 mencement of the thing, indifferent to its progression and 

 termination, and drag in all that follows under the head of 

 the beginning; whence with an overweening haste and 

 impatience, they break off their train of thought in the 

 midst. For in what they say of bodies giving way at the 

 impelling force, there is something ; but why the motion 

 should continue after the urging body is withdrawn, and 

 consequently the necessary alternative of the weaker and 

 stronger body mingling is at an end, of this they say 

 nothing, not sufficiently apprehending the scope of their 

 own observations. 



Others, however, more attentive and steady in investi 

 gating, having marked the force of the air in winds and 

 the like instances, which is capable of throwing down trees 

 and towers, have supposed that the force which urges and 

 accompanies projectiles, after the first impulsion, ought to be 

 referred to the air accumulating and rushing in behind the 

 body in motion, by the impulse of which that body is borne 

 along, like a ship in the expanse of water. And such per 

 sons, at least, do not quit their subject, but carry their 

 thought to its conclusion; yet they, nevertheless, do not 

 attain to the truth. The cause in reality is this. The 

 principal motion seems to be in the parts of the volant 

 body itself, which parts being imperceptible to vision, on 

 account of their extreme tenuity, escape the notice of men, 

 not sufficiently attentive to their subject, and passing it 

 over with a cursory glance. But to one who gives it a 

 sounder examination, it is clearly evident, that the harder 



