258 Mr. Mallet on the Pliysical Conditions 



221. It would be foreign to our immediate end to pursue this subject Bere 

 at any great length, however interesting and important, and with one familiar 

 instance we must dismiss it. 



The well-known operation by which a blacksmith breaks, cold, over his 

 anvil, a bar of the toughest iron that can be had, and whose fibre is all longi- 

 tudinal, consists in " nicking" one or both opposite sides of the bar, at the 

 required point of its length, to a very small depth with a chisel having an edge 

 formed to a very obtuse angle, generally about 90°, and driven into the sub- 

 stance of the bar, by blows from a sledge with great velocity. When this is 

 done, a bar of moderate size, so tough and fibrous that at every other place it 

 is capable of being bent sharply double without fractvire, may be broken across 

 at the " nicked" place, often by bending over one's knee ; always by a few light 

 blows transversely on the anvil. 



When the fracture is exposed, the iron is found at the " nick" to be short 

 and crystalline ; the crystals are on the whole arranged transversely to the bar's 

 length. Their facets are largest and most transverse, just at the sinus of the 

 angle of the nick, and either no sign of the longitudinal fibre constituting the 

 structure of every other part of the bar is visible, or occasionally some portion 

 of the section at the side or part most remote from the nick, or in the centre 

 between both, is still visible. 



Now what happens in this is rendered obvious by the following diagram 

 (Plate vl), in which Fig. 1 represents the side of such a bar at the nicked 

 place, and the change of direction there of the crystalline axes. But, what 

 internal forces have acted on them to produce this ? 



Looking at Fig. 2, it will be seen that the driving the edge of the wedge- 

 shaped chisel into the substance of the bar has produced compressions, whose 

 pressures are propagated in directions perpendicular to the faces of the wedge 

 or of the nick that is the express copy of it. These pressures, in the directions 

 CO, co', are resisted, and finally equilibrated by the elastic compression of 

 the longitudinal crystals in the directions of theu- principal axes, namely, /o, /V ; 

 and the resultants of these mutual pressures meet in the substance of the bar, in 

 the directions or^ o'r', which are those of maximum internal strain ; but in the 

 space between these, cd, approaching the angle of the nick, and perpendicular to 

 these resultants, are the lines of minimum pressure. Now these are the new 



