[ ^^7 J 
T atmofphere moft near 
the furface of the earth ^ for there, befides the dired 
tnany refledions. Moreover, the earth 
itlelf being heated, communicates of its heat to the 
neighbouring air. 
The higher regions having only the dired rays of 
the lun palling through them, are comparatively very 
• Hence the cold air on the tops of mountains 
and fnow on fome of them all the year, even in the 
torrid zone. Hence hail in fummer. 
If the atmofphere were, all of it (both above and 
helow) always of the fame temper as to cold or heat, 
then the upper air would always be rarer than the 
o\\er, becaulc the prelTure on it is lefs ^ confequently 
iightei, and therefore would keep its place. 
But the upper air may be more condenfed by cold, 
than the lower air by prellure. The lower more 
expanded by heat, than the upper for want of pref- 
nire.^ In fuch cafe, the upper air will become the 
heavier, the lower the lighter. 
The lower region of air being heated and expand - 
ed, heaves up and lupports, for fome time, the 
colder heavier air above, and will continue to fupport 
it while the equilibrium is kept. Thus water is fup- 
ported in an^ inverted open glafs, while the equili- 
brium is maintained by the equal prefiure upwards 
of the air below ; but the equilibrium by any means 
breaking, the water defcends on the heavier fide, and 
the air rifes into its place. 
The lifted cold heavy air over a heated country, 
becoming by any means unequally fupported, or un- 
equal in its weights, the heaviefi; part defcends fird, 
and the reft follows impetnoufly. Hence gufts after 
heats, and hurricanes in hot climates. Hence the 
H h 2 air 
