Page:Popular Science Monthly Volume 37.djvu/179

Rh of the light through the transparent portions. But by obscuring the entire surface of * the globe, and then cutting even a very modest design upon the background so prepared, the result is much more effective. The transmitted light, from its subdued character, is also more agreeable. The cutting is done in the so-called "mud-box"—a designation which has arisen from the fact that the spent sand or mud from the cutting of heavier articles is here utilized.



These processes are all purely mechanical. They depend upon the direct friction between the glass and the abrading powder, or between the glass and the cutting stone, as in the case of the smoothing process. It is possible, however, to bring about this grinding action by less direct pressure. One of these indirect methods—the sand-blast—deserves particular mention, both because of its commercial importance and because of its ingenuity. Some years ago there was published a book which pointed out, with more or less cunning, a prototype in nature for nearly all our mechanical devices. The author did not, I believe, mention the sand-blast, but he might well have done so, for it is a direct imitation, though perhaps an unconscious one, of a process which Nature has been using very effectively ever since the first blast of wind carried the earliest sand-grains against the Eozoic rocks. This natural sand-blast has done not a little in altering the appearance of the face of the earth. In the Rocky Mountains there are many curiously sculptured rocks in the comparatively rainless districts, which owe their carving almost entirely to this