202 Obituary Notices of Fellows deceased. 



to one, as in Poisson's theory of the constitution of matter. He refers 

 to indiarubber as hopelessly violating Poisson's condition. Stokes' 

 position, powerfully supported by Lord Kelvin, seems now to be 

 generally accepted. Otherwise, many familiar materials must be 

 excluded from the category of elastic solids. 



In 1846 he communicated to the British Association a Report 

 on Recent Researches in Hydrodynamics. This is a model of what 

 such a survey should be, and the suggestions contained in it have 

 inspired many subsequent investigations. He greatly admired the 

 work of Green, and his comparison of opposite styles may often recur 

 to the reader of mathematical lucubrations. Speaking of the 

 Reflection and Refraction of Sound, he remarks that " this problem 

 had been previously considered by Poisson in an elaborate memoir. 

 Poisson treats the subject with extreme generality, and his analysis is 

 consequently very complicated. Mr. Green, on the contrary, restricts 

 himself to the case of plane waves, a case evidently comprising nearly 

 all the phenomena connected with this subject which are of interest in 

 a physical point of view, and thus is enabled to obtain his results by 

 a very simple analysis. Indeed Mr. Green's memoirs are very remark- 

 able, both for the elegance and rigour of the analysis, and for the 

 ease with which he arrives at most important results. This arises in a 

 great measure from his divesting the problems he considers of all 

 unnecessary generality ; where generality is really of importance he does 

 not shrink from it. In the present instance there is one important 

 respect in which Mr. Green's investigation is more general than Poisson's, 

 which is, that Mr. Green has taken the case of any two fluids, whereas 

 Poisson considered the case of two elastic fluids, in which equal 

 condensations produce equal increments of pressure. It is curious, 

 that Poisson, forgetting this restriction, applied his formulae to the 

 case of air and water. Of course his numerical result is quite 

 erroneous. Mr. Green easily arrives at the ordinary laws of reflection 

 and refraction. He obtains also a very simple expression for the 

 intensity of its reflected sound. . . ." As regards Poisson's work in 

 general there was no lack of appreciation. Indeed, both Green and 

 Stokes may be regarded as followers of 'the French school of 

 mathematicians. 



The most cursory notice of Stokes' hydrodynamical researches 

 cannot close without allusion to two important memoirs of somewhat 

 later date. In 1847 he investigated anew the theory of oscillatory 

 waves, as on the surface of the sea, pursuing the approximation so as 

 to cover the case where the height is not very small in comparison with 

 the wave-length. To the reprint in " Math, and Phys. Papers " are 

 added valuable appendices pushing the approximation further by a 

 new method, and showing that the slopes which meet at the crest of 



