870 Handbook of Nature-Study 



tion of the atmosphere along any meridian from the north to the south pole. 

 The equator would then become the warm center and the poles the cool 

 margins. We would then expect to find a belt of low pressure around the 

 world near the equator because of the high temperature, and high pressure 

 at the poles because of the low temperature. We would, also, expect to 

 find ascending currents at the equator; upper currents flowing from the 

 equator toward the poles; descending currents at the poles, and surface 

 winds blowing from the poles toward the equator. Let us now test our 

 theory by actual facts and see how far they are in accord. 



The chart, Fig. 2, represents the normal, or average, pressure at sea 

 level for the world, and if our theory is in accord with the facts, we should 

 find a belt of low pressure all around the world near the equator, with areas 

 of high pressure at the poles. Let us examine the chart. Beginning at the 

 equator, and bearing in mind that the normal pressure is about 30.00 inches, 



1%? 4; Ecr %" ir 



Fig. 3. Diagram showing air currents along any meridian. 



we find irregular lines, representing pressures of 29.90 inches slightly 

 below normal around the world on both sides of the equator. Between 

 these lines we find pressure as low as 29.80. It is, therefore, evident that 

 there is a belt of low pressure around the world near the equator, as antici- 

 pated. Let us look for the high pressure at the poles. We have compara- 

 tively few observations near the poles, but the line nearest the south pole 

 is marked 29.30 inches, a surprisingly low pressure, much lower even than 

 the low belt at the equator, and just the reverse of what we expected to find. 

 When we look at the north pole we find that the pressure is not so low as at 

 the south pole, but still below normal and about as low as at the equator. 

 Going north and south from the equator we find that the pressure increases 

 gradually up to about latitude 35 in the northern hemisphere and to about 

 latitude 30 in the southern, after which it decreases toward the poles. So 

 there are two well marked belts of high pressure circling the globe ; the one 

 about 3 5 north, and the other about 30, south of the equator. May it not 

 be significant that these belts of high pressure coincide so nearly with the 

 margins, or banks, of the air rivers mentioned on page 867 ? 



Thus far our theory does not accord very well with the facts. True, we 

 found the low pressure at the equator as anticipated ; but we also found low 

 pressure at the poles, where the reverse was expected ; and the high pressure 

 that we anticipated at the poles, we found not far north and south of the 

 equator. We will, therefore, have to discard our theory, or reconstruct it 

 to accord with the facts. Let us reconstruct Fig. i , and mark the pressure 

 on the line representing the earth's surface along any meridian to accord 

 with the facts as they appear on Fig. 2. 



The above diagram now represents the true pressure along any meridian, 

 as determined by actual observations, and we cannot escape the conviction 

 that the requirements as to temperature and pressure at the warm center 



