SEA EOUTES, CALM BELTS, AND VARIABLE WINDS. 339 



still more probable from this circumstance : All the great deserts 

 are in the northern hemisphere, and the land suiface is aUo 

 much greater on our side of the equator. The action of the sun 

 upon these unequally absorbiug and radiating surfaces in and 

 behind, or to the northward of the north-east trades, tends to 

 check these winds, and to draw in large volumes of the atmo- 

 sphere, that otherwise would be moved by them, to supply the 

 partial vacuum made by the heat of the sun, as it pours down its 

 rays upon the vast plains of burning sands and unequally heated 

 land surfaces in our overheated hemisphere. The north-west 

 winds of the southern are also, it may be inferred, stronger than 

 tlie south-west winds of the northern hemis]3here. 



G33. Why the south-east trades are the stronger. — That the south- 

 east trade-winds should, as observations (§ 343) have shown, be 

 stronger than the north-east trade- winds, is due in part also to 

 the well-established fact that the southern (§ 446) is cooler than 

 the northern hemisphere. The isothermal lines of Dove show 

 that the air of the south-east is also cooler than the air of the 

 north-east trade-winds. Being cooler, the air from tjie cool side 

 would, for palpable causes, rush with greater velocity into the 

 equatorial calm belt than should the lighter air from the warmer 

 or northern side. The fact that the air in the lower latitudes of 

 the southern hemisphere is the cooler will assist to explain many 

 other contrasts presented by the meteorological conditions on 

 023posite sides of the equator. Plate XIII. shows that we have 

 more calms and more fogs, more rains and more gales, with more 

 thunder, on our side than on tiie other, and that the atmosphere 

 preserves its condition of unstable equilibrium with much more 

 imiformit}-, being subject to changes less frequent and violent on 

 the south side of the equator than on the north side. 



634. Their uniformity of temperature. — The highest summer 

 temperature in the world is to be found in the extra-tropical 

 countries of the north. The greatest extremes of temperature are 

 also to be found among the valleys of the extra-tropical north. 

 In the extra-tropical south there is but little land, few valleys, 

 and much water ; consequently the temperature is more uniform, 

 changes are less sudden, and the consequent commotions in the 

 air less violent. 



635. The mean place of the equatorial calm belt. — Following up 

 these facts with their suggestions, we discover the key to many 

 phenomena whicb before were locked up in " the chambers of 



