] 48 FRAGMENTS OF SCIENCE. 



dum. Nay, glass may be depolished by tlie 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 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 

 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 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 moder- 

 ate, must, to produce fire, be in the highest degree con- 

 centrated. 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 local- 

 ized. 



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,* who is the 

 inventor of the sand-blast. To his spontaneous kindness I 

 am indebted for some beautiful illustrations of his process. 

 In one thick plate of glass a figure has been worned out to 

 a depth of three-eighths of an inch. A second plate, seven- 

 eighths of an inch thick, is entirely perforated. In a circu- 



*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 McOlellan, now a civil engineer, whom 

 I had the honor of frequently meeting in New York, is a most emi- 

 nent 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. 



