Page:Popular Science Monthly Volume 8.djvu/318

304 Connected with a wooden box, supported on a shelf, as here indicated, is a flexible rubber tube, which in turn is attached at its lower end to an iron tube, that rises through the floor of a miniature wagon. This wagon rests on the roof of a box through which a slit is cut in the direction shown. Through this slit the iron tube passes, projecting into the box below. It is to the structure of this metal tube, or gun, as it is called, that attention is specially directed. As shown in the



section at the right, it consists of two tubes, the one leading down from the box and conveying the sand being smaller, thus allowing of an annular space between it and the lower section. Into this lower section, and at a right angle to it, the blast of air is admitted from a suitable reservoir. The sand falling down, as shown by the upper arrow, enters the lower tube at a point below that at which the air is admitted. Having passed below the limits of its conducting-tube, it receives an extra impulse from the air-current that also is passing downward, and by it is projected with greater force upon the hard substance below. In addition to the advantage gained by this new impulse, it will also be seen that the blast serves another purpose in blowing away the sand, so soon as its work is done, and thus leaving the surface below clean and in a condition to be the more readily acted upon by the succeeding blasts. The purpose of the wagon is merely to admit of the tube being moved forward and backward along the line of the plate to be engraved, the lateral movement of the plate being effected by a suitable device not here shown. This plate is inclosed in a box, for the reason that the falling grains of sand, while they chip away the surface of the plate, are also broken up and