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THE POPULAR SCIENCE MONTHLY 



Connectecl with n wooden box, suiiported on a shelf, as here indi- 

 cated, 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 





Fig. 3. The Tilghman Sand-blast Machine. 



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 ^ae lower section. Into this lower sec- 

 tion, and at a right angle to it, the blast of air is admitted from a suit- 

 able 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 admit- 

 ted. Having passed below the limits of its conducting-tube, ij re- 

 ceives an extra impulse from the air-current that also is passing 

 doAvnward, and by it is projected with greater force npon 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 leav- 

 ing the surface below clean and in a condition to be the more readily 

 acted upon by the succeeding blasts. The purpose of the wngon 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 pow- 



