NOVUM ORGANUM 205 



comes nearly transparent. This at least may be considered 

 certain, that light is always either received and transmitted 

 or reflected by an even surface. 



Again, let the required nature be the motion of projec 

 tiles (such as darts, arrows, and balls) through the air. The 

 school, in its usual manner, treats this very carelessly, con 

 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, that two bodies cannot exist in one place, or there 

 would be a penetration of dimensions. 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 sup 

 porting the impression, but pushing themselves forward in 

 succession to ease it. Fracastorius, and nearly all those 

 who have entered into any refined inquiry upon the sub 

 ject, 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. Among 

 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 power 

 ful 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- 



