222 THE POPULAR SCIENCE MONTHLY. 



depolished by the impact of fine shot ; the lead in this case bruising 

 the glass before it has time to flatten and turn its 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 this 

 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 mo- 

 tion. By turning otf 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 concentration of en- 

 ergy 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 localized. 



But we can go far beyond the mere depolishing of glass ; indeed, 

 I have already said that quartz-sand can wear a hole through corun- 

 dum. This leads me to express my acknowledgments to General 

 Tilghman, 1 who is the inventor of the sand-blast. To his sponta- 

 neous kindness I am indebted for these beautiful illustrations of his 

 process. In this plate of glass you find a figure worked out to a 

 depth of f- of an inch. Here is a second plate & of an inch thick, en- 

 tirely perforated. Here, again, is a circular plate of marble, nearly 

 half an inch thick, through which open-work of the most intricate and 

 elaborate description has been executed. It would probably take 

 many days to perform this work by any ordinary process ; with the 

 sand-blast it was accomplished in an hour. So much for the strength 

 of the blast ; its delicacy is illustrated by this beautiful example of 

 line-engraving, etched on glass by means of the blast. 



This power of erosion, so strikingly displayed when sand is urged 

 by air, will render you better able to conceive its action when urged 

 by water. The erosive power of a river is vastly augmented by the 

 solid matter carried along with it. Sand or pebbles caught in a river- 



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. It is obvious that a na- 

 tion with these tendencies can have no desire for war. 



