170 THE CIVIL ENGINEERS OF THE BODY 



TABLE XXVI. 



AVERAGE STRENGTH OF MATERIALS 



(in kga. per sq. mm.). 

 Material. Crushing Strength. Tensile Strength. 



Steel - 145 100 



Wrought Iron 20 40 



Cast Iron 72 12 



Wood (fibrous tissue) 2 4 



Bone - - 1,3-16 9-12 



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, 

 e.g. cast iron. A further factor has to be taken into account, 

 viz. elasticity. In the following table, borrowed from " Growth 

 and Form " by Prof. D'Arcy Thompson, are given the loads 

 which various fibres and various wires were found capable sus- 

 taining just at their elastic limit. 



TABLE XXVII. 



Stress at Elastic Limit , t o^t'etchm* 

 (grams per sq. mm.). per mille) 



Secale cereale - 15-20 4-4 



Lilium auratwn 19 .7-6 



Phormium tenax 20 13-0 



Papyrus antiquorum - 20 15-2 



Molina coerulea 22 11-0 



Pincenectia recurvata - 25 14-5 



Stress at Elastic Limit , . o 

 (grams per sq. mm.). mt 



Copper Wire 12-1 1-0 



Brass 13-3 1-35 



Iron 21-9 1-0 



Steel 24-6 1-2 



Comparison of these figures with those of the previous table 

 will demonstrate the suitability of fibrous tissue to withstand a 

 stretching strain and yet retain a very high degree of elasticity. 



The engineer plans his structures to give the maximum strength 

 with the minimum weight. Nature is equally economical. 

 Examination of the girders holding a roof will show that they are 

 of two shapes. Those running from wall to wall having a cross- 

 section like the capital letter "I." This girder is a tie and has 

 to withstand a stretching force. How has the engineer arrived 

 at this form ? Figure 29 represents a beam of square section. 

 When such a beam is loaded midway between its supports it 

 is slightly bent to give a concave upper surface. The upper 



