202 THE GRAMMAR OF SCIENCE 



is essentially geometrical, or ideal. It depends on the 

 conception of a straight line fixed in the body and fixed 

 in space about which the body turns. It further involves 

 the conception of the body turning through a certain 

 angle, but an angle Euclid tells us is the inclination of 

 two lines. Thus our description of change of aspect 

 depends upon the conception of lines existing in the 

 rigid body. It is entirely a conceptual description, but 

 like the idea of point-motion, it again serves as a power- 

 ful means of discriminating and classifying our experiences 

 of perceptual motion. 



§ 5. — On CJiange of Fonn, or Strain 



Thus far we have analysed the motion of our man 

 ascending the staircase by considering the motion of an 

 ideal point of him, and then treating him as a rigid body 

 turning about this point, or changing its aspect. It only 

 remains for us to consider how, when the point is in any 

 given position and the man has any given aspect, we may 

 remove the condition of rigidity, and describe how he can 

 move his limbs about, change his form, or alter the 

 relative distances of his parts. This change of form is 

 technically termed strain, and its description and measure- 

 ment forms the third great division in the conceptual 

 motion of bodies. Now we cannot in this work enter 

 into a technical discussion of how strain is scientifically 

 described and measured, but for our present purposes we 

 must ascertain whether the theory of strains deals, like that 

 of the translation of a point and that of the rotation of a 

 rigid body, with conceptual ideas. 



There are two fundamental aspects of strain which 

 most of us consciously or unconsciously recognise. These 

 are change of size without change of shape, and change 

 of shape without change of size. Take a thin hollow 

 india-rubber ball and blow more air into its interior. 

 This will increase its size without necessarily changing its 

 shape. It was spherical in shape and remains spherical 

 in shape, only it is larger. We conceive the ball 



