\ 



Professor Forbes on the Colours of the Atmosphere. 4-19 



grove ! and throughout the works, the original lights are 

 working their way, from the heavier colour. 



I remain, Sir, yours, &c., 



Wm. Trull. 



LXIII. The Colours of the Atmosphere considered with re- 

 ference to a previous Paper " On the Colour of Steam 

 tender certain circumstances." By James D. Forbes, Esq., 

 F. R. SS. L. S,' Ed., Professor of Natural Philosophy in the 



Ufiiversity of Edinburgh.* 



IN the following Paper, it is proposed to illustrate more fully 

 the hint explanatory of certain atmospheric colours, given 

 in a notice of the remarkable red hue of condensing steam, 

 communicated on the 21st January. Since that time, I have 

 examined with care the principal authors who have adverted 

 to the subject of the colour of the sky generally, and of the 

 redness of sunset in particular ; and since, in the course of 

 that research, I have found much to confirm, and little to 

 modify, the view which I have already taken of the subject, I 

 hope that the present Paper may be considered as a fit ap- 

 pendix to my former experimental notice. It will be recollected 

 that in it I stated the singular fact, that steam does not pass 

 at once from the state of invisible pellucid vapour to that of a 

 misty white cloud, such as issues from the spout of a tea-kettle; 

 but that an intermediate stage occurs, in which it is coloured, 

 even very highly, giving to transmitted light a hue varying 

 from tawny yellow up to intense smoke- red. I then observed, 

 that, since this phjenomenon does not require steam of high 

 tension for its production, it is very probable that the tints of 

 sunset and of artificial lights seen through certain fogs, may 

 be owing to the absorptive action of watery vapour in this 

 critical condition. 



Eberhard, a writer of more than sixty years ago, states that 

 the multitude of opinions of authors on the colour of the sky 

 alarmed him when he came to analyse them ; and as, since 

 his time, these have perhaps been doubled, some idea may be 

 formed of the labour required to collect and classify the scat- 

 tered notices which are to be found in special treatises, acade- 



* From the Transactions of the Royal Society of Edinburgh, vol. xiv., 

 part 2, p. ?>i^> ; having been read before the Society on the 4th of February 

 1839. The previous Paper, read before the same body on the 21st of 

 January, will be found in L. and E. Phil. Mag. for February, pres. vol. 

 p. 121. A letter on the same subject addressed to Professor Forbes, by 

 Mr. Webster, Sec. Inst, C. E., appeared in our number for March, p. 184, 

 2E2 



