674 ON FOEM AND MECHANICAL EFFICIENCY [ch. 



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 compression 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, hke 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 combined. The case 

 of a loaded beam is a famihar 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 reason- 

 ably 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 con- 

 dition its lower fibres are being 

 stretched, or are undergoing a 

 tensile stress, while its upper 

 -pia. 331. fibres are undergoing compres- 



sion. It follows that in some 

 intermediate layer there is a "neutral zone," where the 

 fibres of the wood are subject to no stress of either kind. 

 In hke manner, a vertical pillar if unevenly loaded (as, for 

 instance, the shaft of our thigh-bone normally is) will tend to 

 bend, and so to endure compression on its concave, and tensile 

 stress upon its convex side. In many cases it is the business of 

 the engineer to separate out, as far as possible, the pressure-hnes 

 from the tension-hnes, in order to use separate modes of con- 

 struction, or even different materials for each. In a suspension- 



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

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



