NIAGARA. 195 



necessary that the sand should be the harder substance 

 of the two; corundum, for example, is much harder 

 than quartz; still, quartz-sand can not only depolish, 

 but actually blow a hole through a plate of corundum. 

 Nay, glass may be depolished by the impact of fine 

 shot; the grains in this case bruising the glass, before 

 they have time to flatten and turn their energy into 

 heat. 



And here, in passing, we may tie together one or 

 two apparently unrelated facts. Supposing you turn 

 on, at the lower part of a house, a cock which is fed by 

 a pipe from a cistern at the top of the house, the col- 

 umn .of water, from the cistern downwards, is set in 

 motion. By turning off the cock, this motion is 

 stopped; and when the turning off is very sudden, the 

 pipe, if not strong, may be burst by the internal im- 

 pact of the water. By distributing the turning of the 

 cock over half a second of time, the shock and danger 

 of rupture may be entirely avoided. We have here 

 an example of the concentration of energy in lime. 

 The sand-blast illustrates the concentration of energy 

 in space. The action of flint and steel is an illustra- 

 tion of the same principle. The heat required to gen- 

 erate the spark is intense; and the mechanical action, 

 being moderate, must, to produce fire, be in the high- 

 est degree concentrated. This concentration is secured 

 by the collision of hard substances. Calc-spar will not 

 supply the place of flint, nor lead the place of steel, in 

 the production of fire by collision. With the softer 

 substances, the total heat produced may be greater 

 than with the hard ones, but, to produce the spark, 

 the heat must be intensely localised. 



We can, however, go far beyond the mere depolish- 

 ing of glass; indeed I have already said that quartz- 

 sand can wear a hole through corundum. This leads 



