To the rarefaction of the air at the equator, and the daily revolution of the earth, is attributed the origin of the Trade-Winds, which blow from the east or a little to the north of east, north of the equator, and east or south of east after we are south of the equator.  The hot current of ascending air is replaced by cold winds from the poles.

But why are we not constantly subject to the action of north winds, which we rarely are?  Because of the diurnal motion of the earth, which at the equator equals one thousand miles an hour, the polar winds in coming down to the equator do not have any such velocity, because there is a less comparative diurnal speed in the higher latitudes.  The air at the poles revolves upon itself without moving forward;—­at the equator, the velocity, as we have mentioned, is enormous.  If, then, says Professor Schleiden, we imagine the air from the pole to be carried to the equator, some time must elapse before it will acquire the same velocity of motion from west to east which is always found there.  Therefore it would remain behind, the earth gliding, as it were, from beneath it; or, in other words, it would have the appearance of an east wind.  Lieutenant Maury adopts the same explanation.  It is, indeed, that of Halley, slightly modified.

The warm air, ascending from the equatorial regions, rushes to the poles to be cooled in turn, sliding over the heavy strata of cold air below.

The northern trade-wind prevails in the Pacific between 2 deg. and 25 deg. of N. Latitude; the southern trade, between 10 deg. and 21 deg. of S. Latitude.  In the Atlantic the trades are generally limited by the 8th and 28th degrees of N. Latitude.  The region of calms lies between these trades, and beyond them are what are styled the Variables.  In the former the seaman finds baffling winds, rain, and storms.  Occasionally, from causes not yet fully explained, north and south periodical winds break in upon them, such as the Northers which rage in the Gulf of Mexico.

There are many curious facts connected with the Trades, and with the Monsoons, or trade-winds turned back by continental heat in the East Indies, the Typhoons, the Siroccos, the Harmattans, land and sea breezes and hurricanes, the Samiel or Poison Wind, and the Etesian.  The Cyclones, or rotary hurricanes, offer a most inviting field for observation and study, and are an important branch of our subject.  But we are obliged to omit the consideration of these topics, to be taken up, possibly, at some other opportunity.  The theory of the Cyclones may be justly considered as original with our countryman, Mr. Redfield.  Colonel Reid, Mr. Piddington, and other learned Englishmen have adopted it; and so much has been settled through the labors of these eminent men, that intelligent seamen need fear these storms no longer.  By the aid of maps and sailing-directions they may either escape them altogether, or boldly take advantage of their outward sweep, and shorten their passages.