30 



SCIENTIFIC THOUGHT. 



20. 



Mechanical 

 difference 

 between 

 light and 

 sound. 



ments ; and he applied his results for the purpose of 

 gaining a new basis for the theory of light. His specu- 

 lations were, however, not confined to this. He had 

 started by studying sound and had shown its analogy 

 with light ; but when he ultimately ventured on the bold 

 assumption of a lateral to-and-fro tremor, he showed 

 where the nature of light differed from that of sound. 

 It was in this : that the tremor of sound was that of an 

 elastic fluid such as air, or of any substance in which the 

 movement is carried forward by alternate compression 

 and expansion. But the phenomena of light seemed to 

 require for their explanation two seemingly incompatible 

 assumptions : first, a substance more subtle than air, incap- 

 able of impeding the motion of matter in it ; and, secondly, 

 a substance having vibrations resembling the tremors 

 of what we term solid bodies, e.g., stretched strings. 

 Young is one of the founders of the theory of elasticity. 1 



1 The history of the theories of 

 elasticity has been written by Isaac 

 Todhunter and continued by Pro- 

 fessor Karl Pearson. A perusal of 

 the earlier portion of the work 

 shows how imperfect were the ideas 

 which existed at the time when 

 Fresnel approached the problem in 

 the interest of the wave theory 

 of light. The greatest mathemat- 

 icians, like Euler, had handled the 

 subject, and had damaged their rep- 

 utation, especially in this country, 

 by serious errors or by conclusions 

 which agreed ill with experience. 

 Young was one of the earliest writers 

 on elasticity in the nineteenth cen- 

 tury ; having given considerable at- 

 tention to the subject in his Lec- 

 tures on Natural Philosophy (de- 

 livered in 1802, published in 1807). 

 He there introduces the modulus of 

 elasticity, a term which, with some 



change of meaning, survives in mod- 

 ern treatises. His name, as well as 

 that of Hooke (" Ut tensio sic vis "), 

 appears accordingly at the portal of 

 the science. Young, though Tod- 

 hunter has a significant remark on 

 his obscurity of style, stands out 

 prominently, if compared with con- 

 temporary writers in this country, 

 by his thorough knowledge of the 

 labours of Continental mathemati- 

 cians, among whom he assigns special 

 merit to Coulomb. In general, Tod - 

 hunter has little to say in. praise of 

 English science in this department 

 during the earlier part of the cen- 

 tury, and he considers the " perusal 

 of English text-books on practical 

 mechanics published in the first 

 half of the century a dispiriting 

 task," in consequence of a " want of 

 clear thinking, of scientific accuracy, 

 and of knowledge of the work ac- 



