THE SAND-BLAST. 



303 



here, again, reference must be made to the original "claim," where it 

 will be found that the operation of the blast is limited to the cutting, 

 grinding, etc., of any hard substance. It may be well to note the sig- 

 nificance of this word hard^ since in it lies the secret of the whole pro- 

 cess. The substance upon which the sand acts must be a hard or brit- 



FiG. 2. Device for etching with Sand. 



tie one, falling or being blown upon which, the angular sand-grains 

 chip away minute portions, till at length the whole surface is reduced 

 or scratched to any desired depth. Thus, if the plate which, as shown 

 in the figure, be a glass one, and the workman wishes to engrave on 

 it a flat design, he has only to protect the portions which are not to 

 be acted upon, by a stencil made from rubber, soft iron, leather, or 

 even paper, since, these substances uq^ being hard or brittle, will not 

 be affected by the descending blows of the sand-grains. This the 

 workman has done, and by this means he has been able to depolish 

 or grind the surface of the plate as indicated. Of the methods of 

 constructing and applying these stencils, their variety and several 

 uses, descriptions will be given as we advance. 



From the use of a simple jet of falling sand, we pass on a step, 

 and in Fig. 3 present the Tilghman Sand-blast Machine, in its original 

 and complete form, all subsequent improvements having been made 

 only with a view to some special form of service. The feature of this 

 device, it will be observed, is the use of a blast of air or steam which 

 shall be made to accelerate the falling of the sand through the tube, 

 and thus cause each grain to act wnth additional force upon the op- 

 posing surface. If the reader will, by the aid of the illustration, ob- 

 serve closely the construction of this simple device, he will be able to 

 comprehend, once for all, not only the novelty of the invention, but 

 also its extreme simplicity. 



