JL826.J Nev;r Scientific Booki. 317 



circumference of the iron, after making the proper allowance for 

 what may be thrown off from the circular cutting iron in its 

 passage through the air, which must be considerable. 



P. S. As evidence of the absence of heat, it is stated in the 

 memoir of MM. Darier and Colladon, that the small particles of 

 steel adhering to the edge of the cutter, " seen through a lens, 

 did not appear as if untempered, and when tried with a file were 

 found as hard as the best tempered steel." 



I have never observed the appearance of the particles, or 

 examined their temper, but have examined the burr raised in 

 cutting a plate of steel, which, before the operation, was suffi- 

 ciently soft to file with ease, but in the operation became hard- 

 ened on the outer edge much harder tlian before, which was 

 evidently caused by the great heat, and by being suddenly 

 cooled by the current of air caused by the motion of the cutter ; 

 the same would be the case with particles disengaged by heat, 

 or when hot, and adhering to the edge of the cutter ; the process 

 of hardening in air is appUed by artists to the hardening of very 

 small drills. — (Silliman's Journal.) 



Article XIV. 

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