In those regions lying beyond the southern tropic westerly winds prevail during the greater part of the year, exactly as we find on this side of the northern tropic.  In the southern hemisphere, and far from the land, the wind may be said to blow from the westward almost as steadily as the Trades do from the eastward.  The great object, therefore, for an outward-bound ship is to get far enough south to ensure this fair wind.  Beyond the latitude of 30 deg., and as far as 40 deg., this purpose will generally be answered.

We are sufficiently familiar in England with the fact of westerly winds prevailing in the Atlantic.  From a list of the passages made by the New York sailing packets across the Atlantic, during a period of six years, it is shown that the average length of the voyage from Liverpool to America, that is, towards the west, was forty days; while the average length of the homeward passage, or that from west to east, was only twenty-three days.  And it may fix these facts more strongly in the recollection, to mention that the passage-money from England to America (in the days of sailing packets) was five guineas more than that paid on the return voyage.

This prevalence of westerly winds beyond the tropics is readily explained by the same reasoning which has been applied to the Trades blowing within them.  The swift moving air of the torrid zone, on being rarefied and raised up, flows along towards the poles, and in a direction from the equator, above the cooler and slower-moving air, which, as I have already described, is drawn along the surface of the earth from the temperate regions beyond the tropics.  When the rarefied equatorial air has travelled some thirty or forty degrees of latitude along the upper regions of the atmosphere towards the poles it becomes cooled, and is ready to descend again, between the latitudes of 30 deg. and 60 deg., to supply the place of the lower air, drawn off towards the equator by the Trade-winds.  But this partially-cooled air falls on a part of the earth’s surface which is moving much more slowly towards the east, in its diurnal rotation, than the air which has descended upon it, and which is still impressed with a great proportion of its eastern velocity due to the equatorial parallels of latitude, where it was heated and raised up.  The necessary consequence of this is, to produce a rapid motion in the air from the west over the earth’s surface; and this, combined with the other motion of the same portion of air, or that which has driven it from the equatorial regions, produces this remarkable prevalence of south-westerly winds in the northern hemisphere, and north-westerly winds in the southern hemisphere, in those districts lying between the latitudes of 30 deg. and 60 deg..