Now consider the candle and the barn door. If we 

 simply press the candle, end-on, against the wood, the 

 candle gives way and breaks up ; in other words, its 

 different parts change their relative positions. But if they 

 are called upon to do this in an extremely short space of 

 time, say the 20,000th part of a second (which is some- 

 where near the mark), they offer so effectual resistance 

 that the candle retains Its form, and the door gives way. 

 But how about the door ? What happens to that ? The 

 answer is best given by the pane of glass, in which, we 

 know, a shot moving at a sufficient velocity will punch a 

 little round hole. Consider what would happen if we 

 simply pressed the shot against the pane until it broke. 

 We should find, if we made delicate measurements, that, 

 before it broke, the whole pane would bend ; which means 

 that the entire mass of the glass was coming to the assis- 

 tance of the part attacked. But in the case of piercing 

 by a shot, the same measurements would not show any 

 movement of the rest of the pane under the impad. That 

 imparl lasts only about the 1 00,000th part of a second, 

 and in this minute space of time the mass of the glass 

 may be said not to move at all, its resistance to movement 

 at the rate of the shot being enormous. The consequence 

 is that the part attacked is unsupported, and gives way — 

 the effect being strictly local. If a punch supplied the 

 place of the shot, and if a die could be made which gave 

 absolutely equal support to the glass round the part at- 

 tacked by the punch, a round hole could be made without 

 splintering. The inertia of the mass of the glass in the 

 case we arc considering exadly fulfils the conditions of 

 such a die, and a round hole is punched. 



78 



