Page:Popular Science Monthly Volume 8.djvu/320

306 large supply-box, into which the sand is elevated by a series of hoppers 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 or 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 preliminary processes are needed, the plates of glass being fed in at the opening indicated on the right, and passing through to be received and delivered at once as ground glass.



When it is desired, however, to engrave figures or designs upon the plates, a special process precedes the grinding. This consists 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