In South Africa, also, we find the eastern mountainous coast-lands covered with giant timber—in striking contrast with the parched savannas or dreary wastes of the interior; and in the South Sea the difference of verdure between the east and west coasts of the Sandwich Islands, the Feejees, and many other groups, never fails to arrest the attention of the mariner.
The trade winds of the northern and southern hemispheres do not, however, blow in one continuous stream over the whole breadth of the tropical ocean, but are separated from each other by a zone or belt of calms, occasioned by their mutually paralysing each other’s influence on meeting from the north and the south-east, and by the attraction of the sun, which, when in the zenith, changes the easterly air-currents into an ascending stream. From this dependence on the position of the sun, it may easily be inferred that the zone of calms fluctuates, like the trade winds themselves, to the north or south, according to the seasons; and that it is far from invariably occupying the same degrees of latitude, or the same width, at all times of the year. In the Atlantic, from the causes previously mentioned, it constantly remains to the north of the line, where its breadth averages five or six degrees; in the Pacific it more generally extends during the antarctic summer, on both sides of the equator.
Besides the intensity of its heat, the zone of calms is characterised by heavy showers, which regularly fall in the afternoon, and are caused by the cooling of the saturated air-columns in the higher regions of the air.
Daily, towards noon, dense clouds form in the sky, and dissolve in torrents of rain under fearful electrical explosions, now sooner, now later, of shorter or longer continuance, with increasing or abating violence, as the sun is more or less in his zenith. Towards evening the vapours disperse, and the sun sets in a clear, unclouded horizon. Thus towns or countries situated within the calms, such as Para, Quito, Bogota, Guayaquil, the Kingsmill Islands, may be said to have a perennial rainy season, as showers fall at every season of the year. To the north and south of the belt of calms, we find in both hemispheres a broad zone, characterised by two distinct rainy seasons, separated by two equally distinct periods of dry weather. The rainy seasons take place while the sun is crossing the zenith, and more or less paralysing the power of the trade winds. Cayenne, Honduras, Jamaica, Pernambuco, Bahia, afford us examples of these well-defined alterations. In Jamaica, for instance (18° N. lat.), the first rainy season begins in April, the second in October; the first dry season in June, and the second in December.
Towards the verge of the tropics follow the zones which are characterised by a single rainy season, but of a longer continuance, generally lasting six months, throughout the summer, or from one equinoctium to another. In these parts, the rainy season is also the warmest period of the year, since here the different height of the sun in winter and summer is already so considerable, that at the time of culmination the clouds and rain are not able to reduce the temperature below that of the clear and dry winter months; while in the zones which are situated nearer to the equator, the rainy season, in spite of the sun’s culmination, is always the coolest.
To sum up the foregoing remarks in a few words: the two rainy seasons, which characterise the middle zone between each tropic and the line, have a tendency to pass into one annual rainy season on advancing towards the tropics, and to merge into a permanent rainy season on approaching the equator. As the sun goes to the north or south, he opens the sluices of heaven, and closes them as he passes to another hemisphere.
Such may be said to be the normal state which would everywhere obtain within the equatorial regions if one unbounded ocean covered their surface, and none of the disturbing influences previously mentioned interfered; but as we find the trade winds so frequently deflected from their course, we must also naturally expect the general or theoretical order of the dry and rainy seasons to be liable to great modifications.
Thus, in the Indian Ocean and in the Chinese Sea, terrestrial influences prevail during the summer which completely divert the trade wind, there called the North-east Monsoon, from its regular path. From the wide lands of south-eastern Asia, glowing, during the summer months, with a torrid heat, the rarefied air, as it rises into the higher regions, completely overpowers the usual course of the trade wind, and changes it into the south-western monsoon, which blows from May to September.
Hence, during the summer months, the saturated sea wind, striking against the western ghauts, brings rain to the coast of Malabar, while the opposite coast of Coromandel remains dry; but the inverse takes place when, from the sun’s declining to the south, the north-east trade wind resumes its sway.
Similar deflections from the ordinary course of the trade winds occur also on the coast of Guinea (5° N. Lat.), in the Mexican Gulf, and in that part of the Pacific which borders on Central America, through the influence of the heated plains of Africa, Utah, Texas, and New Mexico, and have a similar influence on the distribution of moisture. Thus the sea monsoon, which prevails during the summer months on the coast of northern Guinea, carries a rainy season over the land as far as the eighteenth or nineteenth degree of northern latitude, and fertilises a vast extent of country which, from its position on the western side of an immense continent, would otherwise have been as naked and barren as the Sahara.