On the whole, we doubt if the rationale of the matter has ever been summed up in so explicit and satisfactory a statement as that upon which Mr. Treadwell has based his very successful and remarkable experiments. Difficult as it is to curtail the brevity of his reasoning, we will make such short extracts from his pamphlet as we can bring within our cramped limits. With regard then to the fact that wrought-iron is much stronger than cast-iron or bronze, he says :
"This is certainly true if we expose the wrought iron to the testing force in one direction only. But all wrought iron is in its structure fibrous, the fibres being more or less distinctly marked, according to the process followed in the manufacture of the iron. In wire it is most clearly apparent, the fibres in some cases being so easily parted that the wire can be split with a knife. In sheets, formed by the rolling-mill, the fibres are arranged in plates or laminae, and these often so slightly adhere one to another that they may be separated like the layers of a pasteboard. With hammered iron the grain, or fibres, are less apparent, and the bars possess, in their different directions, greatere equality of strength. By comparing the various operations of wire-drawing, rolling, and hammering, we are led to the conclusion that the fibres are always formed in the direction in which the iron is extended, and the cohesion is least amongst the atoms which are spread over each other. All that is here said of iron is equally true of steel, the cohesive force of which, however, exceeds in an essential degree that of iron. Cast iron and bronze, on the contrary, are of equal strength in all direction ; their structure appearing as an aggegate of grains, assuming the form of crystals, often apparent to the naked eye. The strength or direct tenacity of these various metals, the wrought iron and steel beingtested in the direction of their fibres, may be taken as follows, for each square inch area of the metal:
| Steel, (English spring.) | 100,000 pounds. |
| Wrought iron, | 65,000 " |
| Bronze, | 30,000 " |
| Cast iron, | 25,000 " |
If, however, the steel or wrought iron be exposed to the testing force in such a ways that the fibres shall be separated laterally, instead of being broken, the strength will rarely be found to exceed that of bronze or cast iron even."