276 Prof. Tyndall on the Absorption and 



vection of the heat of the glass plates would produce an effect 

 exactly the same as that of true absorption. By allowing the 

 air in my tube to come into contact with the radiating plate, I 

 have often obtained a deflection of twenty or thirty degrees, — the 

 effect being due to the cooling of the plate, and not to absorp- 

 tion. It is also certain that had I used heat from a luminous 

 source, I should have found the absorption of 033 per cent, 

 considerably diminished. 



§ 8. I have now to refer briefly to a point of considerable inter- 

 est as regards the effect of our atmosphere on solar and terres- 

 trial heat. In examining the separate effects of the air, carbonic 

 acid, and aqueous vapour of the atmosphere, on the 20th of last 

 November, the following results were obtained : — 



Air sent through the system of drying-tubes and through the 

 caustic-potash tube produced an absorption of about . . 1. 



Air direct from the laboratory, containing therefore its car- 

 bonic acid* and aqueous vapour, produced an absorption of . 15. 



Deducting the effect of the gaseous acids, it was found that 

 the quantity of aqueous vapour diffused through the atmosphere 

 on the day in question, produced an absorption at least equal 

 to thirteen times that of the atmosphere itself. 



It is my intention to repeat and extend these experiments on 

 a future occasion f; but even at present conclusions of great 

 importance may be drawn from them. It is exceedingly proba- 

 ble that the absorption of the solar rays by the atmosphere, as 

 established by M. Pouillet, is mainly due to the watery vapour 

 contained in the air. The vast difference between the tempera- 

 ture of the sun at midday and in the evening, is also probably 

 due in the main to that comparatively shallow stratum of aqueous 

 vapour which lies close to the earth. At noon the depth of it 

 pierced by the sunbeams is very small; in the evening very 

 great in comparison. 



The intense heat of the sun's direct rays on high mountains is 

 not, I believe, due to his beams having to penetrate only a small 

 depth of air, but to the comparative absence of aqueous vapour 

 at those great elevations. 



But this aqueous vapour, which exercises such a destructive 

 action on the obscure rays, is comparatively transparent to the 

 rays of light. Hence the differential action, as regards the heat 

 coming from the sun to the earth and that radiated from the 

 earth into space, is vastly augmented by the aqueous vapour of 

 the atmosphere. 



* And a portion of sulphurous acid produced by the two gas-lamps 

 used to heat the cubes. 



t The peculiarities of the locality in which this experiment was made 

 render its repetition under other circumstances necessary. 



