346 A SHORT HISTORY OF SCIENCE 



ularly when they are informed that it was written by a young man who 

 has been obliged to obtain the little knowledge he possesses, at such 

 intervals and by such means as other indispensable avocations which 

 offer but few opportunities of mental improvement, afforded.' Where 

 in the history of science have we a finer instance of that sort of modesty 

 which springs from a knowledge of things ? 



Just as the theories of astronomy and geodesy originated in the 

 needs of the surveyor and navigator, so has the theory of elasticity 

 grown out of the needs of the architect and engineer. From such 

 prosaic questions, in fact, as those relating to the stiffness and the 

 strength of beams, has been developed one of the most comprehensive 

 and most delightfully intricate of the mathematico-physical sciences. 

 Although founded by Galileo, Hooke, and Mariotte in the seventeenth 

 century, and cultivated by the Bernoullis and Euler in the last 

 century, it is, in its generality, a peculiar product of the present 

 century. It may be said to be the engineers' contribution of the cen- 

 tury to the domain of mathematical physics, since many of its most 

 conspicuous devotees, like Navier, Lame, Rankine, and Saint-Venant, 

 were distinguished members of the profession of engineering. . . . 



The theory of elasticity has for its object the discovery of the 

 laws which govern the elastic and plastic deformation of bodies or 

 media. In the attainment of this object it is essential to pass from 

 the finite and grossly sensible parts of media to the infinitesimal and 

 faintly sensible parts. Thus the theory is sometimes called molec- 

 ular mechanics, since its range extends to infinitely small particles 

 of matter if not to the ultimate molecules themselves. It is easy, 

 therefore, considering the complexity of matter as we know it in the 

 more elementary sciences, to understand why the theory of elasticity 

 should present difficulties of a formidable character and require a 

 treatment and a nomenclature peculiarly its own. . . . 



It is from such elementary dynamical and kinematical considera- 

 tions as these that this theory has grown to be not only an indis- 

 pensable aid to the engineer and physicist, but one of the most at- 

 tractive fields for the pure mathematician. As Pearson has remarked, 

 'There is scarcely a branch of physical investigation, from the plan- 

 ning of a gigantic bridge to the most delicate fringes of color exhibited 

 by a crystal, wherein it does not play its part.' It is, indeed, funda- 

 mental in its relations to the theory of structures, to the theory of 

 hydromechanics, to the elastic solid theory of light, and to the theory 

 of crystalline media. 



