158 ELEMENTS OF SCIENCE 



of Mexico. The great chain of the Andes runs from 

 north to south very near the west coast of South America. 

 Very different would be the effect of the trade winds 

 did the Andes bound the eastern instead of the western 

 coast of that continent. 



These various modifications of aerial and ocean 

 currents, cause the temperature of different places of 

 the earth's surface and atmosphere and the degrees of 

 their humidity (in other words, their climates) to vary 

 independently of their degree of latitude, i.e., their 

 distance from the equator. Different, indeed, are the 

 climates of mild Cornwall and frigid Newfoundland 

 (though the latter is south of the former), and of Bordeaux 

 and Halifax (in Nova Scotia) both of nearly the same 

 degree of latitude. Lines which connect places of similar 

 temperatures are termed isothermal lines. They are 

 much and irregularly curved, and therefore widely differ 

 from the circles which mark degrees of latitude. 



The evaporation of water over the earth's surface 

 causes the atmosphere, as before pointed out,* to contain 

 a greater or less quantity of aqueous vapour, sometimes 

 even to become saturated, that is, to contain the greatest 

 quantity it can possibly hold. The rapid condensation 

 of this vapour produces rain. 



When air laden with vapour blows from districts with 

 a cooler climate to another which is warmer, it acquires a 

 still greater capacity for aqueous vapour, and the result 

 is that the .clouds (which, as before said, are masses of 

 minute particles of water), being dissipated into steam, 

 vanish altogether. 



On the other hand, when winds saturated with vapour 

 pass into a cold region, torrents of rain may result. 



* See ante, p. 148. 



