CONSTANT WINDS.—_The Trade Winds._—The violent contrast between the temperature of the equator and the poles is well known, and from the vast area included within the tropics ascending currents of rarefied air are incessantly rising and being as incessantly replaced by a rush of cold air from the poles to the equator. Were the earth stationary, this interchange would be of the simplest kind; on arriving within the influence of the ascending equatorial current the air from the poles would be carried to the higher regions and turning over would proceed to the poles, and, becoming cold and dense in traversing the higher stratum, would descend and resume its course _ad infinitum_. The revolution of the earth on its axis changes all this: the first effect is that the air at the equator is borne along with the earth at the rate of seventeen miles a minute from west to east, a rate which diminishes at 60° of latitude to one-half that velocity, until at the poles it is nothing; consequently a _slow_ north wind flowing to the equator is continually passing over places possessing a higher velocity than itself, and not immediately acquiring that velocity, there is according to the law of the composition of forces a compromise effected resulting in a north-east wind. In a similar manner the same process in the southern hemisphere results in a south-east wind. These winds have acquired the name of _Trade Winds_ on account of the facilities afforded to navigation by their constancy. The _North Trades_ occur in the Atlantic between 9° and 30° and in the Pacific between 9° and 26°. The _South Trades_ occur in the Atlantic between lat. 4° N. and 22° S. and in the Pacific between latitude 4° N. and 23-1/2° S. These limits extend northward with the sun from January to June, and southward from July to December. Parallel to the equator and extending between 2° and 3° on each side is a broad belt, where the north and south trades neutralize each other, producing what is called the “_Region or Belt of Calms_.” Though wind is absent, thunderstorms and heavy rains are of daily occurrence. When Humboldt ascended Teneriffe the trade wind was blowing at its base in the usual direction, but on arriving at the summit he found a strong wind blowing in the opposite direction. Observation has shown that this upper current prevails north and south of the equator, and that, after passing the limit of the trade winds, it descends to form the south-_west_ winds of the north temperate zone and the north-_west_ winds of the south temperate zone; the _westing_ being due to the same cause as the _easting_ in the regular trades, viz., the rotation of the earth on its axis. These winds are called the Return Trades, but are not equal in constancy to the regular trade winds.