Sy, eS ee oT — 
relative to the Earth’s Surface. 37 
calm belt, must be a plane inclining toward the equator above. 
And since, according to (§ 21), the atmosphere near the earth’s 
surface cannot have an eastward motion, unless it also has a 
motion toward the poles, this plane near the earth’s surface 
must nearly coincide with the one which separates the atmo- 
sphere moving towards the poles from that moving towards the 
equator, in the trade wind regions, and hence the latter must 
peak gradually becomes north of it; and hence the southwest 
wind, which always prevails at the top, gradually descends 
lower on the sides of the peak until it reaches the base. Hence, 
when this plane reaches its most southern position, in the latter 
i of winter, the southwest wind prevails at both the base and 
the top. 
24. The depression of the atmosphere at the poles and at the 
equator, and the accumulation near the tropics, may be explained 
in eo agiesea manner by means of the principle in (§ 5) that when 
a body moves in any direction in the northern hemisphere, it is 
deflected to the right, and the contrary in the southern. The 
atmosphere towards the poles having an eastward motion, the de- 
ecting force arising from it causes a pressure towards the equator, 
and the motion near the equator being westward, the pressure is 
towards the poles; and hence there must be a depression at the 
poles and at the equator, and an accumulation near the tropics. 
Since this deflecting force is as cos 6, it is small near the equator; 
and, consequently the depression there is small. _ 
25. According to the preceding tables of barometric pressure, 
there is more atmosphere in the northern than in the southern 
hemisphere. Says Sir James Ross, “ the cause of the atmosphere 
being so very much less in the southern than in the northern 
hemisphere remains to be determined.” This is very satisfacto- 
rily accounted for by the preceding principle; for as there is 
rebern — land, with high nega ranges, in the northern 
m | 4 y i é e } a 
