132 ISLAND LIFE 



air allows such dark heat-rays to pass freely, yet the 

 aqueous vapour, and carbonic acid in the air intercept and 

 absorb them. But the air thus warmed by the earth is in 

 continual motion owing to changes of density. It rises up 

 and flows off, owing to the greater weight of the cooler air 

 which forces it up and takes its place ; and thus heat can 

 never accumulate in the atmosphere beyond a very mode- 

 rate degree, the excessive sun-heat of the tropics being 

 much of it carried away to the upper atmosphere and 

 radiated into space. Water also is very mobile ; and 

 although it receives and stores up a great deal of heat, it 

 is for ever dispersing it over the earth. The rain w^hich 

 brings doAvn a certain portion of heat from the atmosphere, 

 and which often absorbs heat from the earth on which it 

 falls, flows away in streams to the ocean ; while the ocean 

 itself, constantly impelled by the winds, forms great cur- 

 rents, which carry off the surplus heated water of the 

 tropics to the temperate and even to the polar regions, 

 while colder water flows from the poles to ameliorate the 

 heat of the tropics. An immense quantity of sun-heat is 

 also used up in evaporating water, and the vapour thus 

 produced is conveyed by the aerial currents to distant 

 countries, where, on being condensed into rain, it gives up 

 much of this heat to the earth and atmosphere. 



The power of water in carrying away heat is well 

 exhibited by the fact of the abnormally high temperature 

 of arid deserts and of very dry countries generally ; while 

 the still more powerful influence of moving air may be 

 appreciated, by considering the effects of even our northern 

 sun in heating a tightly-closed glass house to far above the 

 temperature produced by the vertical sun of the equator 

 where the free air and abundance of moisture exert their 

 beneficial influence. Were it not for the large projDortion 

 of the sun's heat carried away by air and water the tropics 

 would become uninhabitable furnaces— as would indeed 

 any part of the earth where the sun shone brightly 

 throughout a summer's day. 



We see, therefore, that the excess of heat derived from 

 the sun at any place cannot be stored up to an important 

 amount owing to the wonderful dispersing agency of air 



