260 Mr. Mallet on the Physical Conditions 



223. A good deal of information on this subject occurs in Mr. Hood's Paper, 

 on the "Changes of Internal Structure of Iron" (Pr. Ins. Civ. Eng., vol. ii. p. 180). 

 He attributes the changes which he describes to the conjoint action of " percus- 

 sion, heat, and magnetism," but without any distinct views or attempt at a 

 united theory. He suggests no solution of the way, nor fixes any limits within ■ 

 which percussion acts; and the conjoint action of heat, and especially of 

 " magnetism," appear perfectly gratuitous ; — words without a physical idea. 



224. Mr. Thorneycroft, also, in a paper on the same subject (Pr. Ins. Civ. 

 Eng., vol. is. p. 295), has collected some interesting facts, though his state- 

 ments seem rather warped by certain preconceived views. 



225. In no paper, however, that the author has seen, is any attempt made to 

 connect all the phenomena of change of crystalline structure in iron at all tem- 

 peratures, with the action of some one recognisable force, such as that which 

 he believes to constitute the true solution and key to all the varied and complex 

 facts noticeable, and which he considers he has been the first to enunciate, 

 namely, the arrangement of the principal axes of the crystals in the lines of 

 minimum pressure within the mass. 



27. — Effects of the Variable Rapidity of the Blow or of the Velocity of appli- 

 cation of the FMpturing Strain, upon the character of Fracture of the same 

 Wrought-Iron. 



226. It has been stated (section 113), that under the rapid stroke of cannon- 

 shot, the longest and most fibrous wrought-iron breaks short and crystalline, 

 like cast-iron. 



About 1842, a number of experiments was made at Woolwich, under the 

 direction of General Dundas, upon the effects of 32 lbs. shot, fired at short 

 distances and at various velocities against wrought-iron targets variously pre- 

 pared to represent sections of the sides of iron ships. 



These experiments were generally understood at the time, to have been urged 

 upon Government by certain promoters of iron ship-building, in the expecta- 

 tion that the results would signally establish the superiority of iron over timber 

 as material for ships of war — a notion obviously founded on the tentative know- 

 ledge of merely practical men of the resistance of tough iron to a slowly acting 

 detrusive force, such as that of a punching press, and not upon any just physical 



