The same happens in the case of the bam door. By 

 virtue of the inertia of the mass of the wood, the attack 

 of the candle is stridly localised, and the part attacked 

 gives way. 



But the velocities we have so far been dealing with 

 are insignificant compared with the velocity of the gases 

 in the explosion of dynamite. These are so enormous that 

 under their influence the particles of air themselves develop 

 an astonishing inertia, cmd air itself acts as a solid. We 

 paste a dynamite Ccirtridge on a slab of stone ; we fire 

 it, and the stone is split. The particles of air have been 

 suddenly required to move at such a velocity that they 

 refuse to do it ; or, at any rate, they respond to so small 

 £Ui extent that they force pressure in the opposite diredion 

 sufficient to split the stone. 



One word as to what happens when a stone makes 

 ducks and drakes. The stone is travelling at a high ve- 

 locity, but this velocity is in a horizontal, not a vertical, 

 diredion ; its vertical velocity is very small. Consequently 

 when it first strikes the water it delivers only a slight 

 vertical blow. It begins to sink, but the moment it begins 

 to sink there is water in front of it — a little hill of water — 

 and this little hill is opposed to the horizontal velocity 

 of the stone, which is very great. As in the other cases, 

 it refuses to move ; it solidifies so to speak, and forms a 

 little solid, smooth, inclined plane up which the stone 

 glides, and flies off the surface of the water again. 



79 



