80 AERIAL CURRENTS. 



ever, we reflect the sun's rays from a metal surface, an 

 exactly contrary result takes place ; the uncovered parts 

 are the first to melt, and the blackened card stands high 

 above the surrounding portion. 



The evidences of science all indicate the sun as the 

 source, not only of that heat which we receive directly 

 through our atmosphere, but even of that which has 

 been stored by our planet, and which we can, by several 

 methods, develope. We have not to inquire if the earth 

 was ever an intensely heated sphere ; this concerns not 

 our question; as we should, even were this admitted, 

 still have to speculate on the origin the primitive source 

 of this caloric. 



Before, however, we proceed to the examination of 

 the phenomena of terrestrial heat, a few of the great 

 results of the laws of radiation and convection claim our 

 attention. 



Nearly all the heat which the sun pours upon the 

 ocean is employed in converting its water into vapour at 

 the very surface, or is radiated back from it, to perform 

 the important office of producing those disturbing influ- 

 ences in the atmosphere, which are essential to the pre- 

 servation of the healthful condition of the great aerial 

 envelope in which we live. 



Currents of air are generally due to the unequal degrees 

 in which the atmosphere is warmed. Heat, by expand- 

 ing, increases the elasticity, and lessens the density, of a 

 given mass. Consequently, the air heated by the high 

 temperature of the tropics, ascends charged with aqueous 

 vapours, whilst the colder air of the temperate and the 

 frigid zones flows towards the equator to supply its place. 

 These great currents of the atmosphere are, independent 

 of the minor disturbances produced by local causes, in 

 constant flow, and by them a uniformity of temperature 

 is produced, which could not in any other way be accom- 

 plished. By these currents, too, the equalisation of the 



