1 68 



THE POPULAR SCIENCE MONTHLY. 



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agency. Still more striking, perhaps, has been the effect of blow- 

 ing 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 blow- 

 ing sand. The action is precisely the same in the atelier, except 

 as to the matter of time. A strong blast of air, charged with par- 

 ticles 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 



arranged that it may 

 be made to enter a 

 sheet-iron box placed 

 so that its upper sur- 

 face shall be at about 

 the level of an ordi- 

 nary 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 rub- 

 ber are then pressed 

 against the opening, 

 and, by means of a 

 pedal, the blast is 

 turned on. In a very 

 ¥§j 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 



Fig. 7.— The Process of Etching. 



