122 SYSTEMS OF FORCES. [CHAP. VII. 



130. Stress and Strain. The tension of a string is an 

 example of stress, and the extension or contraction of a spring is 

 an example of strain. The six components of strain in a body, 

 referred to in Article 121, are quantities in terms of which the 

 extensions and contractions of all the lines joining particles of a 

 body which are near together can be expressed. The law that the 

 tension is proportional to the extension is the particular case for 

 elastic strings of a general law connecting the stresses and strains 

 in a body. It is of almost universal application when the strains 

 are sufficiently small. 



