THE SAND-BLAST. 



505 



derecL And it is that this dust may not interfere with the health and 

 comfort of tlie workmen that the w'hole is confined in a closed box. 



Before describing the several methods by which, through the aid 

 of specially-prepared stencils, the surfaces to be treated are exposed 

 to the action of the blast, we will direct attention to certain of the 

 more recent forms of the machines, all embodying the same general 

 principles, but so modified as to adapt them to the special service for 

 which they are intended. 



Foremost among these devices is the large machine, by the aid of 

 which flat plates are ground or engraved. 



Fig. 4. Machine for engeaving Flat Plates. 



The distinctive feature of this machine is the substitution of a long, 

 narrow slit for the tube; through this the sand falls or is blown in a 

 thin sheet. Referring to Fig. 4, we fiml. the machine composed of a 



VOL. Tin. 20 



