XOVUM OKGANUM. 155 



sidering it enough to distinguish it by the name of violent 

 motion, from that which they term natural, and as far as 

 regards the first percussion or impulse satisfies itself by its 

 axiom, &quot; that two bodies cannot exist in one place, or there 

 would be a penetration of dimensions.&quot; With regard to 

 this nature we have these two crossways. The motion 

 must arise either from the air carrying the projected body 

 and collecting behind it, like a stream behind boats or the 

 wind behind straws ; or from the parts of the body itself 

 not supporting the impression, but pushing themselves for 

 ward in succession to ease it. Fracastorius, and nearly all 

 those who have entered into any refined inquiry upon the 

 subject, adopt the first. Nor can it be doubted, that the 

 air has some effect, yet the other motion is, without doubt, 

 real, as is clear from a vast number of experiments. Amongst 

 others we may take this instance of the cross : namely, 

 that a thin plate or wire of iron rather stiff, or even a reed 

 or pen split in two, when drawn up and bent between the 

 finger and thumb, will leap forward. For it is clear, that 

 this cannot be attributed to the air s being collected behind 

 the body, because the source of motion is in the centre of 

 the plate or pen, and not in its extremities. 



Again, let the required nature be the rapid and powerful 

 motion of the explosion of gunpowder, by which such vast 

 masses are upheaved, and such weights discharged as we 

 observe in large mines and mortars ; there are two cross- 

 ways before us, with regard to this nature. This mo 

 tion is excited, either by the mere effort of the body ex 

 panding itself when inflamed, or by the assisting effort 

 of the crude spirit, which escapes rapidly from fire, and 

 bursts violently from the surrounding flame as from a prison. 

 The school, however, and common opinion, only consider 

 the first effort. For men think that they are great philo 

 sophers, when they assert that flame, from the form of the 

 element, is endowed with a kind of necessity of occupying 

 a greater space, than the same body had occupied when in 

 the form of powder, and that thence proceeds the motion 

 in question. In the mean time they do not observe, that 

 although this may be true, on the supposition of flame being 

 generated, yet the generation may be impeded by a weight 

 of sufficient force to compress and suffocate it ; so that no 

 such necessity exists as they assert. They are right, indeed, 

 in imagining that the expansion, and the consequent emis 

 sion or removal of the opposing body, is necessary if flame 

 be once generated ; but such a necessity is avoided, if the 

 solid opposing mass suppress the flame before it be gene- 



