card be flipped from under a penny, when balanced on 

 the finger, without disturbing the penny ? Why, pasted 

 upon a block of stone, with no other backing but the air, 

 does a dynamite cartridge split the stone block when it 

 is exploded ? 



We could ask more, but perhaps these questions cire 

 enough. 



It is common knowledge, that the ^riklng force of a 

 moving body is proportional to the square of its velocity. 

 If, of two similar projectiles, the velocity of one is twice 

 that of the other, it will strike with four times the force ; 

 if three times, then a blow will be nine times as energetic, 

 and so on. But this law equally applies to the body 

 struck. The greater the velocity of the moving body or 

 projectile, the greater the resistance of the body struck 

 to having that velocity communicated to it. It also applies 

 to the particles of the body struck, and of the projectile 

 itself, which particles in their turn resist relative move- 

 ment — i.e., changing their positions among themselves — 

 with a force proportional to the square of the velocity of 

 impad. 



Under this law liquids, and even gases, if the velocity 

 be high enough, v/ill act as solids ; and the principle will 

 be found to explain all the paradoxes under notice. 



Take the simplest first. In the card and penny trick, 

 if we pull the card away somewhat slowly, we pull the 

 penny with it — the card communicates its own velocity 

 to the penny. But if we pull or flip the card away ten 

 times as fast as before, the resistance of the penny to as- 

 sume such increased rate of movement is 100 times as 

 great, and it apparently remains undisturbed on the finger. 



77 



