866 



surface. It therefore failed to embrace the laws of those motions 

 which depend upon the tangential forces called into play by sliding, 

 and was accordingly inapplicable to many important cases of fluid mo- 

 tions ; such for example as the motions of water in pipes and canals. 

 M. Navier was the first to free the theory of hydrodynamics from 

 this embarrassing restriction, and to give the corrected equations of 

 fluid motion, friction being taken into account. In this he was 

 followed by M. Poisson, in a memoir published in the 'Journal de 

 I'Ecole Polytechnique ; ' and the subject has recently been ably 

 handled by Mr. Stokes, in a paper published in the Cambridge 

 Philosophical Transactions, 1847. In the same paper, the author has « 

 treated also of the theory of motions of elastic solids, and has cor- 

 rected some of the speculations of Poisson upon the subject. 



The theory of the motion of elastic solids is connected with, and 

 taken in its most general form, embraces, that of the luminiferous 

 ether ; and accordingly we must date from the period of Navier's 

 investigations, the large and rapid generalizations which the wave 

 theory of light has received from the hands of Cauchy, Green, and 

 M'Cullagh ; and the same theory (motions of elastic media) in the 

 hands of Mr. Haughton (Transactions of the Royal Irish Academy, 

 1849), based upon the comprehensive method of Lagrange, and freed 

 from all that is arbitrary and hypothetical, has enabled its author to 

 compare together from a higher point of view the theories of light 

 just referred to, to explain the grounds of their resemblance, and to 

 point out their essential distinctions. 



The connection which has just been referred to, between the theory 

 of the motion of elastic solids, and that of the luminiferous ether, 

 was long ago dimly perceived, and has ever been regarded as a dif- 

 ficulty in the hypothesis of undulations. Upon this curious subject 

 we may refer to some interesting remarks by Mr. Stokes (Cambridge 

 Philosophical Transactions, 1847) in a memoir already referred to, 

 which go far to remove the difficulty. It is probable, as he remarks, 

 that the tangential force called into action by a relative displacement 

 of the molecules, exists in all bodies ; and there are good grounds 

 for believing that these displacements, in the case of the luminiferous 

 ether, are much within the limit at which the molecules would as- 

 sume a new permanent arrangement, and thus exhibit the property 

 of fluidity. 



Before leaving this subject, we must not omit to notice the new 

 treatment of the wave theory of light in the hands of Prof. Challis 

 (Cambridge Philosophical Transactions, 1847). In this theory the 

 author regards the ether as a continuous fluid substance, and applies 

 to it the usual equations of hydrodynamics. It is assumed to be of 

 the same uniform density and elasticity, within and without the 

 medium ; and the diminished velocity of propagation is supposed to 

 be due to the obstacles which the atoms of the medium oppose to 

 the free motion of the etherial particles. A polarized ray in this 

 theory is that in which the condensation of the ether is unsymmetri- 

 cal relatively to the axis of the ray. 



Instruments are essential in the purely experimental sciences ; and 



