398 



Mr. R. C. Nichols on the Resistances 



whether lateral action alone was sufficient to account for the 

 whole of the extra strength which had been observed*. 



The use of cast iron as a material for beams and girders 

 has been so much discredited, and to a great extent super- 

 seded by that of wrought iron and steel, that the practical 

 importance of this question has greatly diminished since the 

 date of Mr. Barlow's investigations. It may, however, be 

 still regarded as one of some theoretical interest, and its true 

 solution as not altogether devoid of practical value. 



No explanation was given by Mr. Barlow of the manner in 

 which his assumed resistance to flexure was supposed to act. 

 It may be observed that, in addition to the ordinary direct 

 resistances to extension or compression, another kind of 

 resistance is known to come into operation when the particles 

 of a body situated in one layer or set of fibres are laterally 

 displaced relatively to those in a contiguous layer or set of 

 fibres, as in the case of torsional or shearing strain. 



The nature of this resistance may be illustrated by the 

 accompanying diagrams. A, B, C (fig. 1) are supposed to 

 represent three contiguous 

 fibres or filaments, a a! a", 

 b V b" > c c <//, the particles of 

 which they consist, in their 

 original relative position. 

 Then when these fibres are 

 displaced, as in fig. 2, the 

 distance between a and b, 

 b and c &c. is increased, while 

 that between a and V, b and 

 c', &c. is diminished. Both 

 these displacements will call 

 into action resistances of a 

 tensile or compressive na- 

 ture, and constitute a shear- 

 ing or a torsional strain ac- 

 cording as the lateral motion 

 is direct or rotatory. 



In the case of direct ten- 

 sile strain (fig. 3), where the 

 displacement of all the par- 

 ticles is relatively similar, a 

 like resistance must exist in 

 consequence of the increased 

 distance between the par- 

 ticles in contiguous filaments. But the resultant of these 



* Sir B. Baker, ' On the Strength of Beams, Columns, and Arches/ p. 10. 



/5>/ 



7*2 



Tig. 4- 



