dered. And it is that this dust may not interfere with the health and comfort of the workmen that the whole 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 engraving 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 find the machine composed of a
