Prof. Stokes on the Principles of Hydrodynamics. 393 



centrated ray of a burning-lens applied for a shorter time. Nor 

 does the direction of motion, as involved in the idea of polariza- 

 tion, whether plane or circular, exert any effect ; but it is the 

 frequency of the periodic impulses that is the sole detennining 

 cause. And the phsenomcna of interference, from the superpo- 

 sition of such small motions, occur exactly as might have been 

 predicted. >'''-"-''' 



The immediate cause assigned for such decompositions is, that 

 a ray forcmg the material particles on which it falls into a state 

 of rapid vibration, it comes to pass in many compound molecules 

 that their constituent atoms can no longer exist together as the 

 same group, because of the impossibility of their being animated 

 by consentaneous or conspiring motions; and dislocation, re- 

 arrangement, or decomposition is the result. 



Universitj-, New York, 

 Januaiy 1, 1851. 



XLIX. On the Principles of Hydrodynamics, in Reply to Professor 

 ' ' - Chaliis. 5y Professor Stokes*. 



AFTER briefly referring to my arguments contained in a 

 fomier paper. Professor Challis remarks : — " To this reason- 

 ing an answer may readily be given. The result is arrived at by 

 introducing a motion of translation of the ivhole mass, w^hich is 

 precisely the kind of motion which it is unnecessary to include 

 in the hydrodynamical ec^uations." By the use of the term un- 

 necessary, it might have been supposed that Professor Challis 

 admitted that it was allowable to include such a motion, which 

 is all that my argument requires. However, a few Hues frnther 

 on we find : " On account of the motion which alters the den- 

 sity, pressure, or mutual distances of the fluid particles being 

 wholly independent of any motion of translation of the whole 

 mass, it is a ])rcliminary step in any hydrodynamical problem to 

 get rid of the latter, which may always be done either by con- 

 ceiving the origin of coordinates to partake of the motion of 

 translation, or by impressing it on the mass in a direction oppo- 

 site to that in which it takes place. The residual motion, or 

 the difference between the motions of the particles and the motion 

 of the origin of coordinates, is all that is considered in hydro- 

 dynamics; and if the axiom under discussion applies to this 

 motion, it posscs.ses the requisite degree of generality." I con- 

 fess this is the first I have heard of the necessity of any such 

 " preliminary step," so far at h^ast as the received equations are 

 concerned. As the statement here alluded to is unsupportedj I 

 iiuj yiom -idi « Coinmunicated by the Author. 



