170 THE CIVIL ENGINEERS OF THE BODY 



TABLE XXVI. 



AVERAGE STRENGTH OF MATERIALS 



(in kgs. 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.V16 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 of^tret 

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



Secale cereale 15-20 4-4 



I/ilium auratum 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 S f tram 



(grams per sq. mm.). < amt p * 



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 



