iSyi.] A New Mechanical Agent. 361 



blast tun, and again mingles with the shower of sand upon 

 the glass. 



By covering parts of the glass surface by a stencil or pat- 

 tern of any tough or elastic material, such as paper, lace, 

 caoutchouc, or oil paint, designs of any kind may be en- 

 graved. 



There is a kind of coloured glass made by having a thin 

 stratum of coloured glass melted or " flashed" on one side of 

 an ordinary sheet of clear glass. If a stencil of sufficient 

 toughness is placed on the coloured side, and exposed to the 

 sand blast, the pattern can be cut through the coloured stra- 

 tum in from about 4 to 20 minutes, according to its thickness. 



The theoretical velocity of a current of air of the pressure 

 of four inches of water, he calculates, is (neglecting friction) 

 about 135 feet per second ; the actual velocity of the sand is 

 doubtless much less. 



If a current of air of less velocity is used, say about 1 inch 

 of water, very delicate materials, such as the green leaves of 

 the fern, will resist a stream of line sand long enough to 

 allow their outlines to be engraved on glass. By graduating 

 the time of exposure with sufficient nicety, so as to allow the 

 thin parts of the leaves to be partly cut through by the sand, 

 while the thicker central ribs and their branches still resist, 

 the effect of a shaded engraving may be produced. 



The grinding of such a hard substance as glass by an agent 

 which is resisted by such a fragile material as a green leaf, 

 seems at first rather singular. The probable explanation is, 

 that each grain of sand which strikes with its sharp angle on 

 the glass pulverises an infinitesimal portion, which is blown 

 away as dust, while the grains which strike the leaf rebound 

 from its soft elastic surface. 



The film of bichromatised gelatine used as a photographic 

 negative may be sufficiently thick to allow a picture to be 

 engraved on glass by fine sand, driven by a gentle blast 

 of air. 



For cutting stone the inventor uses steam as the impel- 

 ling jet ; the higher the pressure, the greater is the velocity 

 imparted to the sand, and the more rapid its cutting effect. 



In using steam of about 100 pounds pressure, the sand 

 is introduced by a central iron tube, about 3-i6th inch bore, 

 while the steam is made to issue from an annular passage 

 surrounding the sand tube. 



A certain amount of suction of air is thus produced, 

 which draws the sand through the sand tube into the steam 

 jet, and both are then driven together through a tube about 

 6 inches long, in which the steam imparts its velocity to 



VOL. VIII. (O.S.) — VOL. I. (N.S.) 3 A 



