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 significance of this word hard since in it lies the secret of the whole process. The substance upon which the sand acts must be a hard or brittle
Fig. 2.—Device for etching with Sand.
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 not 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 with additional force upon the opposing surface. If the reader will, by the aid of the illustration, observe 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.
