] 48 fRA GMENTS Off SCIENCE. 
dum. 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 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 Tilghrnan,* 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 worked 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. Tilghraan 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 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. 
