Page:Popular Science Monthly Volume 3.djvu/232

222 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 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 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 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 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 these beautiful illustrations of his process. In this plate of glass you find a figure worked out to a depth of ⅜ of an inch. Here is a second plate ⅞ of an inch thick, entirely 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