202 REPORTS ON THE STATE OF SCIENCE.—1914. 
investigations which have been made deal almost exclusively with the 
more restricted case of two-dimensional stress and strain, or of stress and 
strain in a thin plate parallel to the faces of the plate itself, a problem 
known to elasticians as that of ‘ generalised plane stress.’ ! 
In these cases two methods have proved available. The first method 
consists in measuring directly the deformations of the body studied, 
by observing the actual distortion of a face of the solid parallel to the 
plane of strain. In practice this may be done by ruling this face into 
squares and observing, with a kathetometer or micrometer, the relative 
shifts of various parts of the network. From these, the extent by which 
the angle at a node of the network has been changed from a right angle 
can easily be found, and this quantity, as is well known, measures the 
shearing strain (or ‘slide,’ according to a terminology followed by many 
writers on elasticity, who reserve the word ‘ shear ’ to denote the shearing- 
stress). 
In this way values of the shearing-strain are obtained at the various 
nodes of the network. Again, the changes of distance between adjacent 
1 
E 
E. h 
E 
alas 
Fra. 1. 
nodes can be found, and from these, if the squares of the network are 
sufficiently small, the extensions at the various nodes, parallel to the 
lines of the net, can be obtained. 
The plane-strain can, therefore, be mapped out over the whole face of 
the solid which is under observation. If this method is to give satis- 
factory results it must be applied to materials where the strains are 
comparatively large. It has been applied with considerable success by 
Professor Karl Pearson (1) and various workers associated with him to 
models of dams constructed of gelatine-glycerine jelly, and in this way 
various results of interest in the theory of masonry dams have been 
obtained, although it cannot be said that the complete system of stresses 
in such dams is yet known with any certainty. In other cases measure- 
ments of the distortions produced in circles described on the face of a 
model have been used to determine the principal strains and their direc- 
tions, as in the experiments of Messrs. Wilson and Gore (2). 
Dr. H. N. da C. Andrade (3) has also employed a block of jelly to 
investigate the distribution of slide in such a block when two of its opposite 
} Love, Theory of Hlastictty, p. 135, 
