| Tensile strength | Crushing strength | |
|---|---|---|
| Steel | 100 | 145 |
| Wrought Iron | ã40 | ã20 |
| Cast Iron | ã12 | ã72 |
| Wood | ãã4 | ãã2 |
| Bone | ãã9ã12 | ã13ã16 |
At first sight, bone seems weak indeed; but it has the great and unusual advantage that it is very nearly as good for a tie as for a strut, nearly as strong to withstand rupture, or tearing asunder, as to resist crushing. We see that wrought-iron is only half as strong to withstand the former as the latter; while in cast-iron there is a still greater discrepancy the other way, for it makes a good strut but a very bad tie indeed. Cast-steel is not only actually stronger than any of these, but it also possesses, like bone, the two kinds of strength in no very great relative disproportion.
When the engineer constructs an iron or steel girder, to
take the place of the primitive wooden beam, we know that he
takes advantage of the elementary principle we have spoken of,
and saves weight and economises material by leaving out as
far as possible all the middle portion, all the parts in the
neighbourhood of the ãneutral zoneã; and in so doing he reduces
his girder to an upper and lower ãflange,ã connected together
by a ãweb,ã the whole resembling, in cross-section, an 
But it is obvious that, if the strains in the two flanges are to be equal as well as opposite, and if the material be such as cast-iron or wrought-iron, one or other flange must be made much thicker than the other in order that it may be equally strong; and if at times the two flanges have, as it were, to change places, or play each otherãs parts, then there must be introduced a margin of safety by making both flanges thick enough to meet that kind of stress in regard to which the material happens to be weakest. There is great economy, then, in any material which is, as nearly as possible, equally strong in both ways; and so we see that, from the engineerãs or contractorãs point of view, bone is a very good and suitable material for purposes of construction. {677}
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