On the Motion of a small Sphere acted on by an Elastic Fluid. 343 



careful comparisons throughout the whole extent of the spectrum, 

 before we can determine absolutely the relative absorbing power 

 of the different constituents of our atmosphere. 



One other inference from the facts here developed is worthy 

 of notice before closing this paper. It has been for some time 

 suspected that the blue colour of the sky was in some way con- 

 nected with the vapour in the atmosphere; and it is a fact of com- 

 mon observation that this colour is more intense during the moist 

 weather of summer than during the dryer weather of winter. 

 The distribution of the aqueous lines through the solar spectrum 

 not only confirms the opinion previously entertained, but also 

 points to the cause of the colour. So far as my observations 

 have extended, the aqueous lines are almost wholly, if not com- 

 pletely, confined to the more refrangible portions of the spec- 

 trum. Here they are found in vast numbers, and I am not 

 positive that they exist anywhere else. If, then, the aqueous 

 vapour absorbs most powerfully the yellow and red rays of the 

 spectrum, the blue colour of the sky is the necessary result. The 

 colour is therefore due to simple absorption, and not to repeated 

 reflexions from the surface of drops of water, as some physicists 

 have supposed. 



As can readily be seen, the aqueous lines of the solar spec- 

 trum present a very wide field for investigation, but one which 

 can only be cultivated under peculiar atmospheric conditions. 

 This paper is only intended to open the subject. I hope to be 

 able to continue the study on every favourable opportunity, and 

 shall take pleasure in communicating, through the pages of this 

 Journal, any future results. 

 Cambridge, Jan. 9, 1866. 



LI. On the Motion of a small Sphere acted upon by the Undula- 

 tions of an Elastic Fluid. By Professor Challis, M.A., 

 F.R.S., F.R.A.S* 



THE hydrodynamical problem which forms the subject of this 

 communication, and which I have now had under consi- 

 deration for a long series of years, is beset with peculiar difficul- 

 ties, arising from the high order of the differential equations by 

 which alone it can be solved, and from the imperfect state of our 

 knowledge of the appropriate rules for drawing inferences from 

 their integrals. The analytical treatment of questions relating 

 to the motion and pressure of fluids leads in general to differen- 

 tial equations containing live variables ; the problem I am about 

 to discuss requires the solution of an equation containing four 

 variables, and in no case can the number of variables be less than 



* Communicated bv the Author. 

 2A2 



