Vol. 65.] OP THE CALIFORNIA^ EARTHQUAKE Or" 1906. 9 



gji Now, it must be borne in mind that the apparently complicated 

 scries of stresses just described is in reality a complete system of 

 which no member can exist alone, and the production of any one of 

 the four, by application of an external force, brings the other three 



Fig. 3. — Diagram showing couples and shearing stresses. 



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into play ; consequently, it is easy to construct a model which 

 will illustrate the effect. The form that I have adopted consists 

 of a block of indiarubber 8 inches long by 4 inches broad, with a 

 slit 4 inches in length cut longitudinally in its centre ; this, as 

 represented in figs. 4 & 5 (p. 10), is enclosed in a loose-jointed wooden 



of external forces, and stress as the force or combination of forces which such 

 a molecule exerts in tending to recover its free condition. Lord Kelvin intro- 

 duced the use of the term stress for the external forces or system of forces by 

 which the deformation is produced ; this is equal, but opposite in direction, to 

 the force involved in Rankine's definition, and, being usually more convenient 

 in practice, has come to supersede it. Shear is a strain consisting of a com- 

 pression in one direction, and an elongation, in the same ratio, in a direction 

 perpendicular to the first. Strain in its popular sense involves a partial 

 rupture of continuity, and results when the strain, in its physical sense, over- 

 comes the molecular cohesion of the substance strained. It is somewhat 

 unfortunate that there should be these two meanings for the same word, as they 

 are in reality contradictory ; the production of a strain in the popular sense is 

 in fact a relief of strain in the physical sense of the word. 



