XVI] THE STRENGTH OF M]A T|E R I A L S 969 



ones; it falls all to pieces unless we clamp it together, as ])est we 

 can, in a more or less clumsy and immobilised way. But in life, 

 that fabric of struts is surrounded and interwoven with a compli- 

 cated system of ties — "its living mantles jointed strong, With 

 ghstering band and silvery thong*": ligament and membrane, 

 muscle and tendon, run between bone and bone; and the beauty 

 and strength of the mechanical construction he not in one part or 

 in another, but in the harmonious concatenation which all the parts, 

 soft and hard, rigid and flexible, tension-bearing and pressure-bearing, 

 make up together j*. 



However much we may find a tendency, whether in Nature or 

 art, to separate these two constituent factors of tension and com- 

 pression, we cannot do so completely ; and accordingly the engineer 

 seeks for a material which shall, as nearly as possible, offer equal 

 resistance to both kinds of strain J. 



From the engineer's point of view, bone may seem weak indeed ; 

 but it has the great 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. The strength of timber varies with the kind, 

 but it always stands up better to tension than to compression, and 

 wrought iron, with its greater strength, does much the same; but 

 in cast-iron there is a still greater discrepancy the other way, for it 

 makes a good strut but a very bad tie indeed. Mild steel, which has 

 displaced the old-fashioned wrought iron in all engineering construc- 

 tions, is not only a much stronger material, but it also possesses, 

 like- bone, the two kinds of strength in no very great relative dis- 

 proportion §. 



* See Oliver Wendell Holmes' Anatomist's Hymn. 



t In a few anatomical diagrams, for instance in some of the drawings in 

 Schmaltz's Atlas der Anatomie des Pferdes, we may see the system of "ties" 

 diagrammatically inserted in the figure of the skeleton. Cf. W. K. Gregory, On the 

 principles of quadrupedal locomotion, Ann. N. Y. Acad, of Sciences, xxii, p. 289, 

 1912. 



X The strength of materials is not easy to discuss, and is still harder to tabulate. 

 The wide range of qualities in each material, in timber the wide differences according 

 to the direction in which the block is cut, and in all cases the wide difference between 

 yield-point and fracture-point, are some of the difficulties in the way of a succinct 

 statement. 



§ In the modern device of "reinforced concrete," blocks of cement and rods 

 of steel are so combined together as to resist both compression and tension in 

 due or equal measure. 



