676 ON FORM AND MECHANICAL EFFICIENCY [ch. 

 Average Strength of Materials {in kg. per sq. mm.). 



Bone 9-12 13-16 



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 "neutral zone"; and in so doing he reduces his girder to 

 an upper and lower "flange," connected together by a "web," 

 the whole resembling, in cross-section, an I or an I . 



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 other's 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 engineer's or contractor's point of view, bone is a very 

 good and suitable material for purposes of construction. 



