464 Prof. Challis on the Source and 



Respecting the condensation represented by cr, we may at once 

 assert that it is neither increased nor diminished by the inci- 

 dence of the waves on the second atom, on the general principle 

 that condensations and rarefactions once generated are never 

 destroyed, but are dissipated by propagation into unlimited 

 space. They may be diverted, expanded, and contracted ; but 

 the total quantity of condensation and rarefaction remains the 

 same. 



Next we have to investigate the effect of the incidence of that 

 part of the velocity V which is represented by the second term, 

 and is unaccompanied by condensation. Neglecting, for the 

 sake of simplicity, the motion of the atom, and supposing the 

 velocity of impact at its surface to be W, and the angle of inci- 

 dence at any point to be 0, the velocity due to the resistance of 

 the surface will be W cos 6. Now, although the incident velo- 

 city was not accompanied by condensation, it is evident that the 

 surface of the atom cannot impress this velocity without pro- 

 ducing pressure, and therefore condensation, since action and 

 reaction must be equal. Consequently there will be, apart from 

 the original condensations of the incident waves, new condensations 

 due to the inertia, and constancy of form, of the atom. This is a 

 very important result as regards the theory of the internal heat 

 of the sun, or of any other body, on the undulatory hypothesis 

 of heat. 



Any vibratory motion which the atom might have, whether 

 caused by extraneous waves, or by the dynamic effect of those 

 under consideration, would produce condensations by impressing 

 the fluid, which would coexist with those due to the atom's re- 

 action, but would not alter the above general inference. 



The part of the value of V which is equal to kug, regarded 

 independently of the accompanying condensation, will operate 

 in the same manner as the other part, and will equally give rise 

 to new condensations by the reaction of the atom. These, how- 

 ever, will generally be of much smaller magnitude than the 

 others. 



Such secondary undulations at the surface of the second atom 

 as those which we found to result, by the reaction of its surface, 

 from undulations emanating from the first, will also result from 

 undulations emanating from all the surrounding atoms. And 

 since we may suppose the atoms to be distributed equally in all 

 directions from any one of them, the secondary undulations of 

 that one will, in the aggregate, be equal at all points of the 

 surface, as was assumed to be the case, without assigning a 

 reason, at an early stage of this argument. Accordingly we are 

 led to the conclusion that the undulations which emanate equally 

 in all directions from each atom, and constitute by their dynamic 



