208 FRAGMENTS OF SCIENCE 



water, from the cistern downward, 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 may be entirely 

 avoided. We have here an example of the concentration 

 of energy in time. The sand-blast illustrates the concen- 

 tration of energy in space. The action of flint and steel 

 is an illustration of the same principle. The heat re- 

 quired to generate the spark is intense; and the mechan- 

 ical action, being moderate, must, to produce fire, be in 

 the highest degree concentrated. This concentration is 

 secured by the collision of hard substances. Gale-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 localized. 



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 express 

 my acknowledgments to General Tilghman, 1 who is the 

 inventor of the sand-blast. To his spontaneous kindness 

 I am indebted for some beautiful illustrations of his proc- 



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

 arts in the United States, is forcibly illustrated by the rapid transfer of men like 

 Mr. Tilghman from the life of the soldier to that of the civilian. General 

 McClellan, now a civil engineer, whom I had the honor of frequently meeting 

 in New York, is a most eminent example of the same kind. At the end of the 

 war, indeed, a million and a half of men were thus drawn, in an astonishingly 

 short time, from military to civil life. 



