640 LOUIS VESSOT KING 



by later tests to be only approximately true. It is found that to 

 produce continuous flow in a plastic solid it is necessary continuously 

 to increase the distorting stress. A simple illustration of this fact 

 is to be noticed in the manner in which a short circular cylinder 

 crushed in a testing machine ultimately breaks down. According 

 to Tresca's theory the surfaces of shear should be cones of semi- 

 vertical angle of 45 , while experiments indicate that the angle is 

 more often in the neighborhood of 55 for a material like cast iron. 1 

 These results have led to a modification of Tresca's hypothesis as 

 already mentioned. The effect of this so-called "resistance to 

 flow" does not appear to have been studied with a view to formu- 

 lating the laws according to which solids may be made to flow 

 continuously. . 



In the field of experimental ballistics the use of the permanent 

 deformation of short copper cylinders to measure the enormously 

 high pressures involved in testing explosives by means of the 

 so-called "crusher-gauge," invented by Noble about 1875, 2 has 

 led to the detailed study of the relation of applied stress and 

 deformation produced in these special circumstances. 3 The results 

 of these observations have recently been studied in detail by Bril- 

 louin. 4 The behavior of copper shows the existence of internal 

 friction analagous to that observed by Adams and Bancroft in the 

 case of various rock specimens. 



In the experiments carried out by the latter investigators the 

 use of nickel-steel jackets of standard thickness to incase the rock 

 specimens subjected to flow is analagous to the use of short cylinders 

 of annealed copper in the crusher-gauges just referred to. In order 

 to obtain the lateral pressure on the specimen corresponding to a 

 given deformation of the nickel-steel jacket, a calibration-curve is 

 obtained by filling the cylinders with tallow. The hydrostatic pres- 

 sures required to give a series of deformations give the required 



1 A. Morley, Strength of Materials (Longmans, Green, & Co., 1908), p. 55. 



2 See Encyclopaedia Britannica, nth ed., article on "Ballistics," for a brief descrip- 

 tion of the crusher-gauge. 



3 Vieille, Memorial des poudres et salpetres (Gauthier-Villars, Paris) ,V, 12-61. 



4 M. Brillouin, "Les grandes deformations du cuivre par ecrasement et par 

 traction," Ann. de Chimie et de Physique, o e serie, II (1914), 489-96. 



