430 Correspondence — Mr. A. F. Griffith. 



tance to shearing stress along a plane is independent of the pressure 

 perpendicular to that plane. With regard to this, however, it must 

 be remembered that the shearing stress spoken of is not employed 

 to overcome friction, but to induce viscous motion ; and it is difficult 

 to see how this can be affected by normal pressure, unless perhaps 

 when the matter is compressible. 



But when we turn to the rest of the violent attack, made on 

 papers which treat of a difficult subject in a sufficiently simple way, 

 one is puzzled to know what in the world can have induced any one 

 who cares for his reputation to commit such egregious blunders to 

 print, even in the form of a criticism. And I hope Prof. Blake will 

 excuse my giving an example or two which will, I think, be 

 sufficient to show him that, in his anxiety to save your fledgeling 

 readers from danger, he had unconsciously done the very opposite. 

 In his remarks on Mr. Fisher's first paragraph, he supposes that 

 we are told to confound " vertical " with " perpendicular to the 

 bedding," whereas in fact Mr. Fisher (after suggesting that by 

 the ordinary artifice of turning a whole area together with the 

 directions of the forces affecting it back through the angle of dip, 

 his formulge, which are based on horizontally stratified areas, will, 

 with the necessary modifications, apply generally^) merely warned 

 his readers to make these modifications. He makes an extraordinary 

 error also when he gives those two instances to show that Mr. Fisher 

 was wrong in stating that the idea of plasticity would be introduced 

 by the assumption that pressure varies as the area on which it acts. 

 On what part of a steam-boiler does the pressure vary as the area, 

 except on those surfaces on which the plastic steam or the plastic 

 atmosphere acts ? And even Prof. Tait, the terror of whose name 

 Prof. Blake invokes in another part of his critique, would be puzzled 

 to determine what is the pressure per unit area exerted by a rigid 

 book on a rigid table ; we used to learn that the pressure so created 

 was indeterminate if exerted at more than 3 points ; but as soon as 

 we " introduce the idea of plasticity," it is obvious that the pressure 

 will then vary as the area. 



So much for the barren labour of criticizing the criticism. If 

 however, you will allow me a little more space, I should like to add 

 a few words on the original papers. Mr. Fisher starts with the 

 assumption that the lack of horizontal support, which seems necessary 

 to account for direct faulting, is probably in many cases due to con- 

 traction on solidification ; and then, as it seems to me, follows out 

 logically the consequences of that assumption. Nowhere can the 

 existence of this proposed cause be more easily ascertained than in 

 the " slurries " of the Cambridgeshire coprolite pits ; though the 

 small vertical pressure exerted by the shallow deposits in these 

 slurries is not, I suspect, sufficient to give rise to faulting. Now no 

 one with any knowledge of mathematics would expect that the 

 equations of motion of plastic solids submitted to forces thus 

 generated, even if they could be obtained, could be integrated ; so 



1 Tour readers will forgive my explaining this at such length. 



