196 FKAGMENTS OF SCIENCE. 



me to express my acknowledgments to General Tilgh- 

 man,* who is the inventor of the sand-blast. To his 

 spontaneous kindness I am indebted for some beautiful 

 illustrations of his process. In one thick plate of glass 

 a figure has been worked out to a depth of f ths of an 

 inch. A second plate, ths of an inch thick, is entirely 

 perforated. In a circular plate of marble, nearly half 

 an inch thick, open work of most intricate and elab- 

 orate description has been executed. It would prob- 

 ably take many days to perform this work by any 

 ordinary process; with the sand-blast it was accom- 

 plished in an hour. So much for the strength of the 

 blast; its delicacy is illustrated by this beautiful ex- 

 ample of line engraving, etched on glass by means of 

 the blast. 



This power of erosion, so strikingly displayed when 

 sand is urged by air, renders us better able to conceive 

 its action when urged by water. The erosive power of 

 a river is vastly augmented by the solid matter carried 

 along with it. Sand or pebbles, caught in a river vor- 

 tex, can wear away the hardest rock; * potholes ' and 

 deep cylindrical shafts being thus produced. An ex- 

 traordinary instance of this kind of erosion is to be 

 seen in the Val Tournanche, above the village of this 

 name. The gorge at Handeck has been thus cut out. 

 Such waterfalls were once frequent in the valleys of 

 Switzerland; for hardly any valley is without one or 

 more transverse barriers of resisting material, over 



* The absorbent power, if I may use the phrase, exerted by 

 the industrial arts in the United States, is forcibly illustrated by 

 the rapid transfer of men like Mr. Tilghman from the life of the 

 soldier to that of the civilian. General McClellan, now a civil 

 engineer, whom I had the honour of frequently meeting in New 

 York, is a most eminent example of the same kind. At the end 

 of the war, indeed, a million and a half of men were thus drawn 

 in an astonishingly short time, from military to civil life. 



