Radiant Heat to Aqueous Vapour. 39 



merits are all perfectly concurrent as regards the action of the 

 aqueous vapour upon radiant heat. 



The power of aqueous vapour being thus established, meteoro- 

 logists may, I think, apply the result without fear. That 10 per 

 cent, of the entire terrestrial radiation is absorbed by the aqueous 

 vapour which exists within ten feet of the earth's surface on a 

 day of average humidity, is a moderate estimate. In warm 

 weather and air approaching to saturation, the absorption would 

 probably be considerably greater. This single fact at once sug- 

 gests the importance of the established action as regards meteo- 

 rology. I am persuaded that by means of it many difficulties 

 will be solved, and many familiar effects, which we pass over 

 without sufficient scrutiny because they are familiar, will have a 

 novel interest attached to them by their connexion with the action 

 of aqueous vapour on radiant heat. While leaving these applica- 

 tions to be made in all their fullness by meteorologists, I would 

 refer, by way of illustration, to one or two points on which I 

 think the experiments bear. 



And first it is to be remarked that the vapour which absorbs 

 heat thus greedily radiates it very copiously. This fact must, I 

 think, come powerfully into play in the tropical region of calms, 

 where enormous quantities of vapour are raised by the sun, and 

 discharged in deluges upon the earth. This has been assigned 

 to the chilling consequent on the rarefaction of the ascending 

 air. But if we consider the amount of heat liberated in the for- 

 mation of those falling torrents, the chilling due to rarefaction 

 will hardly account for the entire precipitation. The substance 

 quits the earth as vapour, it returns to it as water ; how has the 

 latent heat of the vapour been disposed of ? It has in great part, 

 I think, been radiated into space. But the radiation which dis- 

 poses of such enormous quantities of heat subsequent to conden- 

 sation, is competent, in some measure at least, to dispose of the 

 heat possessed prior to condensation, and must therefore hasten 

 the act of condensation itself. Saturated air near the surface of 

 the sea is in circumstances totally different from those in which 

 it finds itself in the higher atmospheric regions. Aqueous vapour 

 is a powerful radiant, but it is an equally powerful absorbent, 

 and its absorbent power is a maximum when the body which 

 radiates into it is vapour like itself. Hence, when the vapour first 

 quits the equatorial ocean and ascends, it finds, for a time, a mass 

 of vapour above it, into which it pours its heat, and by which 

 that heat is intercepted and in part returned. Condensation in 

 the lower regions of the atmosphere is thereby prevented. But 

 as the mass ascends it passes through successive vapour-strata 



