208 



THE CIVIL ENGINEERS OF THE BODY 



muscular work by coming into play in the intervals between 

 successive shocks (Marey). 



Examination of Fig. 48 will reveal the fact that bodies with an 

 elasticity like rubber act under stress in exactly the opposite way 

 to bodies like the metals. The metals are at first resistant to the 

 distorting force, and lengthen only slightly till the " yield point " 

 is reached, when they lengthen rapidly with little or no increase 

 of applied force and then suddenly rupture. Rubber, on the other 



hand, at first stretches easily, 

 and then enters on a stage 

 in which it offers greatly 

 increased resistance to the 

 pull and lengthens very 

 little, as the applied force 

 increases, till it breaks. 

 Almost similar graphs could 

 be prepared for compressive 

 and shearing stresses. 



(2) Nature of Structure. 

 A moment's thought will 

 convince one that a quite 

 different structure is re- 

 quired to meet strains of the 

 stretching and of the thrusting varieties, (a) The material used 

 in the l^uilding of struts to withstand thrust must have a high 

 crushing limit, while that going to form ties requires high resistance 

 to stretching. In the following table drawn up by Sir Donald 

 MacAlister, are given the approximate values of the crushing and 

 tensile strengths of some building materials. 



Fig 



INTENSITY OF ^THESi 



48. — Tensile stress-strain furves for rubber and 

 annealed steel wire (on ditfcient scales). 



TABLE XXXI 

 Average Strength of Materials 



A glance at this table is sufficient to show that a material which may make 

 a very good strut may make a very poor tie. This is very clear if we consider 

 such a substance as cast iron, which may withstand a compressive force of 

 40 to 50 tons per square inch before it breaks, but may be pulled to bits by 

 about one-sixtli of that force. It would be quite suitable for struts, but useless 

 for forming a sound tie-bar. On the other hand, bone is almost as well able 

 to resist distortion by pulling as by pressing. If anything, it requires a greater 



