TRADE-WINDS--STORMS--THEIR EFFECTS--MONSOONS--THEIR VALUE--LAND AND SEA BREEZES--EXPERIMENTS--HURRICANES--THOSE OF 1801--ROTATORY STORMS--THEIR TERRIBLE EFFECTS--CHINA SEAS--HURRICANE IN 1837--WHIRLWINDS--WEIGHT OF ATMOSPHERE--VALUE OF ATMOSPHERIC CIRCULATION--HEIGHT OF ATMOSPHERE. Before proceeding to speak of the power and the dreadful effects of wind, it is necessary to say a word or two about the trade-winds. It is supposed that the "trades" derived their name from the fact of their being favourable to navigation, and, therefore, to trade. They consist of two belts of wind, one on each side of the equator, which blow always in the same direction. In the last chapter it was explained that the heated atmosphere at the equator rises, and that the cooler atmosphere from the poles rushes in to supply its place. That which rushes from the south pole is, of course, a south wind, that from the north pole a north wind; but, owing to the Earth's motion on its axis from west to east, the one becomes a north-east, the other a south-east wind. These are the north-east and the South-east "trades." They blow regularly--sometimes gently, sometimes fiercely--all the year round. Between the two is a belt of calms and changeable breezes, varying from 150 to 500 miles broad-- according to the time of the year--where there are frequent and violent squalls, of very short duration, accompanied with heavy rains. This region is called by seamen the "doldrums," and considerable trouble and difficulty do ships experience in crossing it. It has already been explained that about latitude 30 degrees, the upper current of wind from the south descends. At the same point the upper current from the north also descends. They cut through each other, and the point where these two cut each other is the northern limit of the north-east trade-winds. The same explanation holds in regard to the southern limit of the south-east trades. In the accompanying diagram the arrows within the circle point out the direction of the north-east and the south-east "trades" between the tropics of cancer and capricorn, and also the counter currents to the north and south of these, while the arrows around the circle show how counter currents meet and rise, or descend, and produce the calm belts. We have hitherto enlarged chiefly on the grand currents of the atmosphere, and on those modifying causes and effects which are perpetual. Let us now turn to the consideration of those winds which are produced by local causes, and the effects of which are partial.