still more probable from this circumstance: All the great deserts are in the northern hemisphere, and the land surface is also much greater on our side of the equator. The action of the sun upon these unequally absorbing 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 atmosphere, 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 the south-west winds of the northern hemisphere.
633. 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 the 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 opposite 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 the other, and that the atmosphere preserves its condition of unstable equilibrium with much more uniformity, 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 which before were locked up in "the chambers of
