agency. Still more striking, perhaps, has been the effect of blowing sand upon the monuments of antiquity. Those who have seen the Obelisk, in Central Park, New York, or have read descriptions of it, will probably recall the fact that on those sides which were originally exposed to the desert wind the hieroglyphics have been entirely worn off by the grinding action of centuries of blowing sand. The action is precisely the same in the atelier, except as to the matter of time. A strong blast of air, charged with particles of sharp, clean sand, will obscure a plain glass surface in the course of a few seconds. It is applied very ingeniously. The design to be traced on the glass is cut out of soft rubber, and the stencil thus formed is held firmly against the surface. The blast of sand-carrying air is secured by means of an exhaust, and is so Fig. 7.—The Process of Etching. arranged that it may be made to enter a sheet-iron box placed so that its upper surface shall be at about the level of an ordinary table. There is a round opening in the top of the box, somewhat larger than the pattern to be ground, but not so large as the sheet of rubber in which it is cut. Glass and rubber are then pressed against the opening, and, by means of a pedal, the blast is turned on. In a very short time, scarcely more than five or ten seconds, the blast is turned off, and the stenciled pattern is found ground on the glass. So quickly does the blast do its work that the capacity of the machine may be said to be limited only by the speed with which the operator can adjust things.
The action of the blast is rather interesting. The soft-rubber stencils will endure many exposures, while the hard flint glass is perceptibly worn away in a few seconds. The reason of this is