o 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 localised.

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. [Footnote: 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 honour 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.] 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
circular plate of marble, nearly half an inch thick, open work of 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, renders us 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
vortex, can wear away the hardest rock potholes' and deep cylindrical
shafts being thus produced. An extraordinary instance of this kind of
erosion is to be seen in the Val Tournanche, above the village of this
name. The gorge at Handeck has been thus cut out. Such wa