through the Earth's Atmosphere. 205 



quited into space, and the sun would rise upon an island held 

 fast in the iron grip of frost. The aqueous vapour constitutes 

 a local dam, by which the temperature at the earth's surface is 

 deepened : the dam, however, finally overflows, and we give to 

 space all that we receive from the sun. 



The sun raises the vapours of the equatorial ocean ; they rise, 

 but for a time a vapour screen spreads above and around them. 

 But the higher they rise, the more they come into the presence 

 of pure space, and when, by their levity, they have penetrated 

 the vapour screen, which lies close to the earth's surface, what 

 must occur ? 



It has been said that, compared atom for atom, the absorp- 

 tion of an atom of aqueous vapour is 16,000 times that of air. 

 Now the power to absorb and the power to radiate are perfectly 

 reciprocal and proportional. The atom of aqueous vapour will 

 therefore radiate with 16,000 times the energy of an atom of 

 air. Imagine then this powerful radiant in the presence of 

 space, and with no screen above it to check its radiation. Into 

 space it pours its heat, chills itself, condenses, and the tropical 

 torrents are the consequence. The expansion of the air, no 

 doubt, also refrigerates it; but in accounting for those deluges, 

 the chilling of the vapour by its own radiation must play a most 

 important part. The rain quits the ocean as vapour ; it returns 

 to it as water. How are the vast stores of heat, set free by the 

 change from the vaporous to the liquid condition, disposed of? 

 Doubtless in great part they are wasted by radiation into space. 

 Similar remarks apply to the cumuli of our latitudes. The 

 warmed air, charged with vapour, rises in columns, so as to 

 penetrate the vapour screen which hugs the earth ; in the pre- 

 sence of space, the head of each pillar wastes its heat by radi- 

 ation, condenses to a cumulus, which constitutes the visible 

 capital of an invisible column of saturated air. 



Numberless other meteorological phenomena receive their 

 solution by reference to the radiant and absorbent properties 

 of aqueous vapour. It is the absence of this screen, and the 

 consequent copious waste of heat, that causes mountains to be 

 so much chilled when the sun is withdrawn. Its absence in 

 Central Asia renders the winter there almost unendurable; in 

 Sahara the dryness of the air is sometimes such that, though 

 during the day "the soil is fire and the wind is flame/' the 

 chill at night is painful to bear. In Australia, also, the ther- 

 mometric range is enormous, on account of the absence of this 

 qualifying agent. A clear day, and a dry day, moreover, are very 

 different things. The atmosphere may possess great visual clear- 

 ness, while it is charged with aqueous vapour ; and on such occa- 

 sions great chilling cannot occur by terrestrial radiation. Sir 



