16 G. F.Becker — Current TJieories of Slaty Cleavage. 



cork and sponge have properties which enable them to be 

 used in approximate illustration of ( negative ) elongation. Mr. 

 Leith not only defines pure shortening, but illustrates it. The 

 illustration, however, shows a strain in which only two of the 

 axes have undergone change, and which is neither more nor 

 less than a ( pure ) shear. Perhaps, then, shortening is only a 

 new term for shear. 



Mr. Leith says that Thomson and Tait call an irrotational or 

 pure shear a " simple " shear. This is incorrect. They desig- 

 nate by " simple " shear the strain I ventured to rename scis- 

 sion, while they call a pure shear simply a " shear."* It was the 

 confusing similarity between the terms for essentially different 

 strains which led to my innovation. In my nomenclature, 

 pure is a qualification of shear which is superfluous except- 

 ing when the reader might possibly forget the definition of 

 scission. 



Farther on, he states that " scission is equivalent to a pure 

 shortening combined with a rotation of the body as a whole. " 

 If the verbal definition is accepted, this is wholly untrue. 

 If the diagram is correct so that shortening = shear, it is 

 true in a sense, because the two processes may each give an 

 equal ellipsoid in the same orientation ; but the boundary con- 

 dition of the two ellipsoids would not be the same, so that if 

 each were elastic their behavior on being set free suddenly 

 would differ fundamentally. The ellipsoid due to scission would 

 not only vibrate about the mean spherical figure, but (because 

 of inertia) spin on its mean axis ; while that due to pure defor- 

 mation followed by rotation would not spin. If the ellipsoid 

 were plastic, that produced by scission would show a fiber or 

 cleavage under proper conditions of rupture or etching. Rolled 

 steel is a case in point ; and when blocks of steel are used to 

 test explosives, the substitution of rolled metal (which has 

 undergone scission) for forged metal (in which, so far as possible, 

 scission is avoided) can be detected at a glance. 



Mr. Leith states that " stress is the action and reaction 

 between two adjacent parts of a body." This definition defines 

 nothing at all. If stress is action, it cannot be reaction. In 

 fact, the phrase sounds like a confused echo of the theorem as 

 to the equilibrium of different stresses embodied in Newton's 

 third law. Stress is simply the total force measured per unit 

 area -which is exerted between contiguous bodies or contiguous 

 parts of a body. It is distinguished from " bodily" forces such 

 as gravity, which are measured per unit volume. 



The author further states that " any possible stress may be 

 regarded as equivalent to three normal stresses whose directions 



*See Ngt. Phil., paragraphs 17.2 and 632. Love employs the same terms, 

 which are in accord with the commonly accepted definitions. 



