These constant winds near the equator have been named trade-winds, because of their importance to commerce. Unknown before the first voyage of Columbus, they filled the minds of his crew with fear that they could never return home, if the wind blew always in one direction. The same gentle wind bore Magellan in his voyage around the world, and caused him to give the name of “Pacific,” or “peaceful,” to the great ocean on our west; and the same steady breezes made the fortune of many a noble galleon in the days when Peru was an Ophir, Mexico an El Dorado, and the Philippine Isles a Tarshish where they took shipping for the distant land of gold. Owing to the fact that the continents intercept the regular trades by reason of their elevation and irregular conformation, and also because of their much greater specific heat, whereby they set in motion many other local currents, the trades are found to begin only a considerable distance to the west of the continents. Yet the influence of the trades is sufficient to make easterly winds the prevailing ones on the great inland plains: as in the Sahara, Arabia, Southern Siberia, and portions of North and South America. It is clear that other nearly constant currents must exist to supply the vacuum that would be otherwise caused by the trades. These are found to the south and north of the trade belts, and, as might be expected, blow nearly in the opposite direction, being descending currents; while the trades, as before stated, are ascending. The column of hot air from the equator starts toward the poles above the trades, while a polar current sets in toward the equator; but as the amount of air displaced at the equator is by far the greatest, much of it can, of course, never reach the poles. On meeting the polar current, the two partially mingle and descend, forming what is called the return trade. This blows, most of the year, to the southeast, the equatorial current prevailing and coming from a region whose easterly rotation is more rapid. At certain seasons of the year, however, the polar current prevails to some extent, though not sufficiently to overcome the eastward trend; so the wind in this belt blows alternately to the southeast and the northeast. Between the region of trades and alternating winds is a belt, on either side of the equator, of calms and variable winds, which shift northward or southward, parallel to the belt of calms between the trades. These two zones, however, are much less clearly defined than the great central one, and are not liable to such extraordinary disturbances.