Page:Physical Geography of the Sea and its Meteorology.djvu/195

Rh trade-winds of that region. To make this clear, see Plate VII., on which I have marked the course of such vapour-bearing winds; A being a breadth or swath of winds in the north-east trades; B, the same wind as the upper and counter-current to the south-east trades; and C, the same wind after it has descended in the calm belt of Capricorn, and come out on the polar tide thereof, as the rain winds and prevailing north-west winds of the extra-tropical regions of the southern hemisphere. This, as the north-east trades, is the evaporating wind. As the north-east trade-wind, it sweeps over a great waste of waters lying between the tropic of Cancer and the equator. Meeting no land in this long oblique track over the tepid waters of a tropical sea, it would, if such were its route, arrive somewhere about the meridian of 140° or 150° west, at the belt of equatorial calms, which always divides the north-east from the south-east trade-winds. Here, depositing a portion of its vapour as it ascends, it would, with the residuum, take, on account of diurnal rotation, a course in the upper region of the atmosphere to the south-east, as far as the calms of Capricorn. Here it descends and continues on towards the coast of South America, in the same direction, appearing now as the prevailing north-west wind of the extra-tropical regions of the southern hemisphere. Travelling on the surface from warmer to colder regions, it must, in this part of its circuit, precipitate more than it evaporates. Now it is a coincidence, at least, that this is the route by which, on account of the land in the northern hemisphere, the north-east trade-winds have the fairest sweep over that ocean. This is the route by which they are longest in contact with an evaporating-surface; the route by which all circumstances are most favourable to complete saturation; and this is the route by which they can pass over into the southern hemisphere most heavily laden with vapours for the extra-tropical regions of that half of the globe; and this is the supposed route which the north-east trade-winds of the Pacific take to reach the equator and to pass from it. Accordingly, if this process of reasoning be good, that portion of South America between the calms of Capricorn and Cape Horn, upon the mountain ranges of which this part of the atmosphere, whose circuit I am considering as type, first impinges, ought to be a region of copious precipitation. Now let us turn to the works on Physical Geography, and see what we can find upon this subject. In Berghaus and Johnston—