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The most remarkable of the regular winds are the trade-winds. The atmosphere at the surface between the tropics is much warmer than in the higher latitudes; and since air expands when heated, the light warm air of intertropical regions perpetually rises, and its place is as perpetually supplied by the colder air from the north and the south. If it were not for the Earth’s rotation, these would be merely north and south winds; but like the equinoctial water-currents, these cool currents of air coming from regions which have not an equal velocity of rotation with the air at the equator, pause and hang back, and thus these aerial currents acquire a westerly direction, forming north-easterly constant winds in the northern hemisphere, and south-easterly in the southern hemisphere.