196 FKAGMENTS OF SCIENCE. 



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, Jths of an inch thick, is entirely per- 

 forated. In a circular plate of marble, nearly half 

 an inch thick, open work of most intricate and 

 elaborate description has been executed. It would pro- 

 bably 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 

 example 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 

 vortex, can wear away the hardest rock ; ' potholes ' and 

 deep cylindrical shafts being thus produced. An extra- 

 ordinary 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 Switzer- 

 land ; for hardly any valley is without one or more 

 transverse barriers of resisting material, over which the 

 river flowing through the valley once fell as a cataract. 

 Near Pontresina, in the Engadin, there is such a case ; 



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 as- 

 tonishingly short time, from military to civil life. 



