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THE POPULAR SCIENCE MONTHLY. 



large supply-box, into which the sand is elevated by a scn-ics of hop- 

 pers attached to a moving belt. From this box the sand falls of its 

 own weight into a second receptacle, which serves also as a receiving- 

 chamber for the air-blast that enters at the right through the large 

 blast-pipe. From this receiver the sand is driven downward through 

 a second slit, and emerges from it with great force. 



At right angles with this slit a series of leather straps cr moving 

 belts serves to convey the polished plate beneath the sheet of falling 

 sand, and it is during the passage of the plate under this sand-sheet 

 that its surface is depolished or ground. As these plates move at the 

 rate of from six to thirty inches a minute, an estimate can be made as to 

 the rapidity with which the work of grinding is effected. When it is 

 desired to merely roughen the whole surface, it is evident that no pre- 

 liminary processes are needed, the plates of glass being fed in at the 

 opening indicated on the right, and passing through to be receiv^ed and 

 delivered at once as ground glass. 



Fig. 5. Plates enrraved by Sand-Blast. 



When it is desired, however, to engrave figures or designs upon 

 the plates, a special process precedes the grinding. This conbists in 

 the designing and attaching of the stencils, and may be described as 

 follows: The glass plate, which it is proposed to ornament with any 

 suitable device, is laid upon the designer's table and covered over its 

 whole surface with a thin sheet of tin-foil. Upon this bright metallic 

 surface the designer sketches his pattern, and then by the aid of a 

 sharp knife-point cuts through the foil along the lines of the pattern. 

 The foil, which indicates the design, is then carefully lifted and re- 



