XVI] THE STRENGTH OF MATERIALS 967 



of the same kind and order, so far simplified and particularised that 

 we may to some extent deal with it, and may possibly even find, 

 in our partial comprehension of it, a partial clue to the principles 

 of causation underlying this whole class of phenomena. 



Before we speak of the form of a bone, let us say a word about 

 the mechanical properties of the material of which it is built*, in 

 relation to the strength it has to manifest or the forces it has to 

 resist: understanding always that we mean thereby the properties 

 of fresh or living bone, with all its organic as well as inorganic 

 constituents, for dead, dry bone is a very different thing. In all 

 the structures raised by the engineer, in beams, pillars and girders 

 of every kind, provision has to be 

 made, somehow or other, for strength 

 of two kinds, strength to resist com- 

 pression or crushing, and strength to 

 resist tension or pulling asunder. The 

 evenly loaded column is designed with 

 a view to supporting a downward 

 pressure^ the wire-rope, like the tendon ^^' 



of a muscle, is adapted only to resist a tensile stress ; but in many or 

 most cases the two functions are very closely inter-related and com- 

 bined. The case of a loaded beam is a familiar one; though, by the 

 way, we are now told that it is by no means so simple as it looks, and 

 indeed that "the stresses and strains in this. log of timber are so 

 complex that the problem has not yet been solved in a manner that 

 .reasonably accords with the known strength of the beam as found 

 by actual experiment!." However, be that as it may. we know, 

 roughly, that when the beam is loaded in the middle and supported 

 at both ends, it tends to be bent into an arc. in which condition 

 its lower fibres are being stretched, or are undergoing a tensile 



* Cf. Sir Donald MacAlister, How a bone is built, Engl. III. Mag. 1884, 

 t Professor Claxton Fidler, On Bridge Construction, p. 22 (4th ed.), 1909; cf. 

 {int. al.) Love's Elasticity, p. 20 {Historical Introduction), 2nd ed., 1906, where the 

 bending of the beam, and the distortion or warping of its cross-section, are studied 

 after the manner of St Venant, in his Memoir on Torsion (1855). How complex the 

 question has become may be judged from such papers as Price, On the structure 

 of wood in relation to .its elastic properties, Phil. Trans. (A), ccvni, 1928; or 

 D. B. Smith and R. V. Southwell, On the stresses induced by flexure in a deep 

 rectangular beam, Proc. U.S. (A), cXLiii, pp. 271-285, 1934. 



