THE WEATHER, AND WEATHER PROPHETS. I5I 



ticity out into space, far above the tops of the highest 

 mountains ; yet, in virtue of its compressibihty, so con 

 densed (comparatively) in its lower strata as that one- 

 third of its total ponderable mass lies within a mile of 

 altitude above the sea-level nearly one-half within two, 

 and nearly two-thirds witliin five miles ; within which 

 latter limit the whole would be contained, were it every- 

 where of the same density as on the surface : so that 

 only about one-third of its total mass is free to range, 

 unimpeded by the crests of the highest Himalaya; and 

 not much more than two-fifths can entirely clear the 

 range of the Andes without pressure ct tergo. In conse- 

 quence, when driven in the state of wind over these or 

 other mountain ranges, it is thrown up into vast ripples 

 or waves, which are propagated thenceforward onwards 

 over indefinite areas of land or sea, and become no 

 doubt the origin of a great part of those casual fluctua- 

 tions of the barometer which give so much trouble to 

 meteorologists. 



(14.) This aerial ocean is not of the same temperature 

 throughout, even in the same climate and over the same 

 tract of country. It is everyv.here warmer near the 

 ground, colder aloft : and at very great heights a most 

 intense cold always prevails ; more intense than that of 

 our severest winters. Hence the snow whicli covers the 

 summits of lofty mountains even in the hottest climates. 

 This relation between the temperatures existing below 

 and alott is not subverted by any amount of mutual 

 admixture of the strata, such as internal movements or 

 ascending currents would produce. On the contrary, 



