118 Prof. TyndalFs Remarks on the 



For this it is which contributes to the maintenance of the beau- 

 tiful green of the British Islands ; for it moderates the burning 

 rays of the sun, as well as prevents the great colds, which only 

 occur with a clear sky and a copious radiation. 



Quite recently Father Secchi in Rome*, and Mr. Cooke at Cam- 

 bridge in America f, have connected the occurrence of certain 

 lines in the solar spectrum, which they have observed accom- 

 panies a high hygrometric state of the atmosphere, with the ab- 

 sorption of heat by aqueous vapour. Apart from the circum- 

 stance that, as shown by the above experiments, heat is extremely 

 little absorbed by transparent aqueous vapour, the absorption of 

 light indicated by the small quantity of dark lines which occur 

 with moist air, compared with the luminous intensity of the whole 

 spectrum, corresponds to by no means so great an absorption of 

 heat as, according to Professor Tyndall, must occur. It might 

 rather be maintained that in itself the small diminution in lumi- 

 nous intensity with moist but perfectly transparent air, is a con- 

 firmation that thermal rays are but little absorbed by such air. 



Notwithstanding the great pains taken to ascribe to trans- 

 parent aqueous vapours a great absorptive capacity for heat, it 

 follows from the experiments adduced, and still more from the 

 phenomena of dew, that this powerful absorption is not due to 

 the transparent, but to nebulous vapours. 



XV. Remarks on the Paper of Professor Magnus. 

 By Professor Tyndall, F.R.S., $c.% 



I SHOULD refrain for the present from making any remark 

 upon the paper of my friend Professor Magnus, did I not 

 fear that my silence might be misconstrued by meteorologists, and 

 that they might be withheld, through a doubt as to their value, 

 from prosecuting observations which I think are sure to expand 

 the boundaries of their science. 



For an abstract of the experiments and reasonings by which 

 each successive objection which has been urged against the action 

 of aqueous vapour on radiant heat has been met, I would refer 

 to the second edition of my work f On Heat/ pages 381 to 421. 

 With the desire there manifested to get at the bottom of the 

 difference between us, I approach the latest objections of Pro- 

 fessor Magnus, regretting only that, being on the point of quit- 

 ting London, I can do no more than jot down a few of the more 



* Comptes Rendus, vol. Ix. p. 379. 



t Proceedings of the American Academy of Aits and Sciences, vol. vii. 

 January 1866. [Phil. Mag. May 1866, p. 337-] 

 X Communicated by the Author. 



