360 A New Mechanical 4 gent- [J u ty> 



practically cutting or depolishing the surface of a plate of 

 glass by a sand blast of very moderate intensity. Various 

 examples of hard substances cut, depolished, and carved into 

 shape, were displayed. In the discussion which followed the 

 presentation of this very remarkable discovery, Mr. Robert 

 Briggs, in his interesting remarks on the subject, took occa- 

 sion to say that it had been long remarked that window 

 glass, exposed to the wind-driven sand, near the sea-shore, 

 soon loses its polish, and cited some other well-known exam- 

 ples of the erosion of surface when exposed to a continued 

 stream of moving particles. When we think of the many 

 such examples, and consider that engineers have had con- 

 tinually to make provision against this well-known cutting 

 effect, it seems surprising that it should not have been turned 

 to some good account before this. 



The following description of the process is taken from a 

 paper by Mr. Coleman Sellers communicated to the "Journal 

 of the Franklin Institute," advance sheets of which we owe 

 to the courtesy of the editor, Dr. W. H. Wahl. 



Mr. Tilghman's attention seems first to have been directed 

 towards cutting stone or hard metal by a jet of sand im- 

 pelled by steam escaping under high pressure. His early 

 experiments were with very high pressure, but as he pro- 

 gressed in the knowledge of the results obtainable with 

 various velocities, a great use for this process seemed to de- 

 velope itself in sand driven by moderate air blasts, and ap- 

 plied to grinding or depolishing glass for ornamental pur- 

 poses. 



For grinding glass he uses a common rotary fan, 30 inches 

 in diameter, making about 1500 revolutions per minute, 

 which gives a blast of air of the pressure of about 4 inches 

 of water, through a vertical tube, 2 feet high by 60 inches 

 long, and 1 inch wide. 



Into the top of this tube the sand is fed, and falling into 

 the air current and acquiring velocity from it, is dashed down 

 against the sheets of glass, which are slowly moved across, 

 about 1 inch below the end of the tube. About 10 or 15 

 seconds' exposure to the sand blast is sufficient to completely 

 grind or depolish the surface of ordinary glass ; so that sheets 

 of it carried on endless belts may be passed under this 

 i-inch wide sand shower at the rate of 5 inches forward 

 movement per minute. In the machine in use for this pur- 

 pose the spent sand is re-conveyed to the upper hopper by 

 elevators, and the dust made by the sand blast (which might 

 otherwise be a source of annoyance to the workman) is drawn 

 back into the fan, and thence passes with the wind into the 



