7O 
the gradual progress in the manufacture since wrought- 
iron began to be used instead of cast-iron. This was 
the first great change, and from it dates a new era in 
this branch of industry. And it was not only a great 
change, but a great advance. Wrought ironis a very 
much superior material to cast-iron, and one which de- 
mands very much more skill in its manufacture. Cast- 
iron is of a granular or crystalline nature ; wrought-iron 
is fibrous: cast-iron is hard ; wrought-iron tough. The 



















NATURE 
[NVov. 24, 1870 
difference between them may be illustrated by the dif- 
ference between glass and wood. One is strong to resist 
a statical strain or pressure, the other to resist a dynami- 
cal strain or blow. There is a vast difference between 
the two kinds of strength. A brick which is at the 
foundation of a lofty factory chimney supports an 
enormous weight, but it would be broken by a blow that 
would not injure a stout walking-stick. 
having that kind of strength which 
Wrought-iron 
resists dynamic force 






































is therefore far preferable to cast-iron for resisting the 
violent and sudden shock of explosives, the most power- 
ful dynamic strain with which man’s art has to grapple. 
It averages three times the dynamic strength of cast-iron, 
that is, it will bear three times as great weight without 
breaking. It will yield sooner ; but when cast-iron yields 
it breaks. In this another great advantage is gained. 
When a cast-iron gun breaks it does so explosively ; it 
Scale %"= 7 Foot. 
Wrought Tron or 
(Showing direction of fibre) 
Wrought Iron Coils (Zor 
breaks up into fragments, and gives no warning, no indi- 
cations of yielding beforehand. But a wrought-iron gun 
shows when its use is becoming dangerous. 
Though this discussion seems rather at variance with 
the plan laid down, yet it ismecessary to have a general 
knowledge of the material used in order to understand 
the method of manufacture. Wrought-iron, while it is so 
much better a material for the construction of heavy 





Fic. 2 
guns, is yet very difficult and expensive to work. The 
«rouzht-iron gun cannot be made as easily as the gun for 
which the molten metal was run into a mould and then 
bored out and finished exteriorly. It requires large fur- 
naces, huge steam-hammers, and skilled workmen to 
give it shape. Before the wonderful appliances of modern 
science and machinery were invented, wrought-iron could 
only be made and workedin comparatively small quan- 
tities. And even now to forge the mass necessary for a' gui 
7,12, or 25 tons weight, would be a most difficult and costly, 
perhaps in the last case an impossible undertaking, No 
doubt there are larger forgings used in large steam ships for 
cranks and shafts, and in other machinery ; but these 
masses of wrought-iron are not heated and hammered 
the whole at once. Separate parts are welded together, 
or successive portions are heated and hammered. It is: 

