356 Mr. H. C. Sorby on the Colour of the Clouds and Sky. 



First, let p = q + ? ' an( i Q De successively equal to r, 2r, 3r ...nr, 

 n being any whole number; and we see by (A) that the re- 

 sultant of p and q and the corresponding diagonal of the par- 

 allellogranij with those forces as sides, coincide whenever the 

 forces have the ratios 2, 3, 4, . . . n. 



Second, let r be the unit of force, and p and q be expressed in 

 terms of that unit which, whatever they are, will always make 

 them as near to whole numbers as we please ; then by the series 



above we have ^r^- = -, and 777- = - 3 whence by (B), 

 DL q' PA r J K ' 



m_p 



AQ q 



The proof for magnitude is the same as before. 



Waterville, Maine, U.S., 

 September 27, 1367. 



XLI V. On the Colon?* of the Clouds and Sky. 

 Byll.C. Sorby, F.R.S.* 



THE appearance of LommePs paper (Pogg. Ann, vol. cxxxi. 

 p. 105), and of the translation in the last Number of 

 this Magazine, induces me to publish an account of what I had 

 written on the same subject. On February 19th of this year I 

 gave a public lecture, and on March 5th I read a paper at the 

 Sheffield Literary and Philosophical Society, on the Colour of the 

 Clouds and Sky, and its connexion with the state of the weather. 

 Within a few days of that date the following abstract was pub- 

 lished in several of the local newspapers. 



" The varied colours of the sky and clouds may be accounted 

 for on very simple principles. The chief of these are that the 

 clear transparent vapour of water absorbs more of the red rays 

 of light than of any others, whilst the lower strata of the atmo- 

 sphere, within no great distance of the surface of the earth, offer 

 more resistance to the passage of the blue rays. This is espe- 

 cially the case at sunrise and sunset, and is probably due to im- 

 purities emanating from the earth. This effect is seen in a very 

 intense form in the case of dark-coloured fogs, through which 

 the red rays penetrate far more readily than the blue rays, so 

 that the sun appears red. This is often due to the influence of 

 a few hundred yards in thickness of such dense fogs ; and the 

 author thinks it probable that the same result would occur if the 



* Communicated by the Author. 



