39^ THE SCIENTIFIC PAPERS OF 



Years ago a great advance was made in gun manufacture by Sir 

 William Armstrong in producing his well-known mode of con- 

 struction. At that time wrought iron was the strongest material 

 practically available for the gun-maker's use, and this was put into 

 the strongest possible form by the construction of coiled rings, 

 which method places the fibre of the metal in the direction of the 

 strain. The Woolwich or Fraser system of gun construction being 

 a modification of the Armstrong system comprises the same mode 

 of putting iron into the condition of greatest strength ; but it is 

 time I think to enquire whether, after the recent advances made 

 in the production of mild steel, which is a material of superior 

 strength, tenacity, and uniformity to iron, the mode of construct- 

 ing ordnance should not be modified to suit these altered 

 circumstances. 



It is important then to appreciate wherein consists essentially 

 the difference between iron and mild steel. 



Mild steel is a metal consisting of 99*75 per cent, of the elemen- 

 tary substance, iron ; and only a quarter per cent, of manganese, 

 carbon, and such impurity as phosphorus and sulphur in the 

 smallest possible quantity ; whereas wrought iron of commerce 

 generally consists of 96 to 97 per cent of metallic iron and be- 

 tween 3 and 4 per cent, of other material, for the most part slag. 

 Now, it seems not a very great matter that in 100 Ib. of iron there 

 should be 3 Ib. of slag, but if we represent this proportion to our 

 eyes we see that it is not such a very inconsiderable quantity, 

 considering that slag is both voluminous and entirely devoid of 

 tensile strength. I hold here two cubes, one of 4^" side, and the 

 other of H" side, representing as nearly as may be the one the 

 metallic iron, and the other the slag which when mixed together 

 form wrought iron. If the slag was mixed up amongst the mass 

 of metallic iron in an irregular way, the strength of the iron would 

 probably be very little more than is due to that of the glassy slag, 

 because filaments of slag might go right across ; but in drawing 

 out the iron, again and again, the little original globules of metal 

 become elongated into strings or fibres of iron, held together by 

 filaments of slag, and thus we get in iron a great apparent increase 

 of strength by drawing it. But even if we draw it out to the 

 utmost, we lose strength to the extent of the sectional area taken 

 up by the slag, and thus get less resisting power than if the pure 



