NIAGARA. 195 



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

 actually blow a bole through a plate of corundum. 

 Nay, glass maybe depolished by tbe 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 column 

 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 impact of the 

 water. By distributing the turning of the cock over half a 

 second of time, the shock and danger of rupture maybe 

 entirely avoided. We have here an example of the con- 

 centration of energy in time. The sand-blast illustrates 

 the concentration of energy in space. The action of 

 flint and steel is an illustration of the same principle. 

 The heat required to generate the spark is intense ; 

 and the mechanical action, being moderate, must, to 

 produce fire, be in the highest 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 loca- 

 lised. 



We can, however, go far beyond the mere depolishing 

 of glass ; indeed I have already said that quartz-sand can 

 wear a hole through corundum. This leads me to ex- 

 press my acknowledgments to General Tilghman, 1 who 



1 The absorbent power, if I may use the phrase, exerted by the 

 Industrial arts in the United States, is forcibly illustrated by the 



