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portion of the tropical and subtropical zones where the trade-winds constantly blow, as the cvaporation must there be cnormous while the quantity of rain is very small. It follows, then, that on the equatorial landsurface there will be a considerable balance of condensation over evaporation which must tend to the general raising of the temperature, and, owing to the condensation being principally at night, not less powerfully to its equalisation.
General Features of the Equatorial Clima!e.--The various causes now enumerated are suflicient to enable us to understand how the great characteristic features of the climate of the equatorial zone are brought about ; how it is that so high a temperature is maintainc during the absence of the sun at night, and why so little effect is produced by the sun's varying altitude during its passage from the northern to the southern tropic. In this favoured zone the heat is never oppressive, as it so often becomes on the borders of the tropics; and the large absolute amount of moisture always present in the air, is almost as congenial to the health of man as it is favourable to the growth and development of vegetation. Again, the lowering of the temperature at night is so regular and yet so strictly limited in amount, that, although never cold enough to be unpleasant, the nights are never so oppressively bot as to prevent sleep. During the wettest months of the year, it is rare to have many days in succession
1 Where the inhabitants adapt their mode of life to the peculiarities of the climate, as is the case with the Dutch in the Malay Archipelago, they enjoy as robust health as in Europe, both in the case of persons born in Europe and of those who for generations hare lired under a vertical siin.
without some hours of sunshine, while even in the driest months there are occasional showers to cool and refresh the overheated earth. As a result of this condition of the earth and atmosphere, there is no check to vegetation, and little if any demarcation of the seasons. Plants are all evergreen ; flowers and fruits, although more abundant at certain seasons, are never altogether absent; while many amual food-plants as well as some fruit-trees produce two crops a year. In other cases, more than one complete year is required to mature the large and massive fruits, so that it is not uncommon for fruit to be ripe at the same time that the tree is covered with flowers, in preparation for the succeeding crop. This is the case with the Brazil nut tree, in the forests of the
Amazon, and with many other tropical as with a few temperatc fruits.
Uniformity of the Equatorial Climate in all parts of the Globe.—The description of the climatal phenomena of the equatorial zone here given, has been in great part drawn from long personal experience in South America and in the Malay Archipelago. Over a large portion of these countries the same general features prevail, only modified by varying local conditions. Whether we are at Singapore or Batavia ; in the Moluccas, or New Guinca; at Para, at the sources of the Rio Negro, or on the upper Amazon, the equatorial climate is essentially the same, and we have no reason to believe that it materially differs in Guinea or the Congo. In certain localities, however, a more contrasted wet and dry season prevails, with a somewhat greater range of the thermometer. This is generally associated with a sandy soil, and a less dense forest, or with an open and more As the heated carth, and everything upon its surface, does not cool so fast when surrounded by moist as by dry air, it follows, that even if the quantity and intensity of the solar rays falling upon two given portions of the eartli's surface are exactly equal, yet the sensible and effective heat produced in the two localities may be very different according as the atmosphere contains much or little vapour. In the one case the heat is absorbed more rapidly than it can escape by radiation ; in the other case it radiates away into space, and is lost, more rapidly than it is being absorbed. In both cases au equilibrium will be arrived at, but in the one case the resulting mean temperature will be much higher than in the other.
Influence of inds on the Temperature of the Equator. - The distance from the northern to the southern tropics being considerably more than three thousand miles, and the area of the intertropical zone more than one-third the whole area of the globe, it becomes hardly possible for any currents of air to reach the equatorial belt without being previously warmed by contact with the earth or ocean, or by mixture with the heated surface-air which is found in all intertropical and sub-tropical lands. This warming of the air is rendlered more certain and more effective by the circumstance, that all currents of air coming from the north or south have their direction changed owing to the increasing rapidity of the eartli's rotational velocity, so that they reach the equator as casterly winds, and thus pass obliquely over a great extent of the heated surface of the globe. The causes that produce the westerly monsoons act in a similar manner, so that on the equator direct north or
thousand feet, as in some of the volcanic mountains of Java and on portions of the Eastern Andes. Beyond the forests both to the north and south, we meet first with woody and then open country, soon changing into arid plains or even deserts which form an almost continuous band in the vicinity of the two tropics. On the line of the tropic of Cancer we have, in America the deserts and dry plains of New Mexico; in Africa the Sahara ; and in Asia, the Arabian deserts, those of Beloochistan and Western India, and further cast the dry plains of North China and Mongolia. On the tropic of Capricorn we have, in America the Grand Chaco desert and the Pampas; in Africa the Kalahari desert and the dry plains north of the Limpopo ; while the deserts and waterless plains of Central Australia complete the arid zone. These great contrasts of verdure and barrenness occurring in parallel bands all round the globe, must evidently depend on the general laws which determine the distribution of moisture over the carth, more or less modified by local causes. Without going into meteorological details, some of which have been given in the preceding chapter, the main facts may be explained by the mode in which the great aerial currents are distributed. The trade winds passing over the ocean from north-east to south-west with an oblique tendency towards the equator, become saturated with vapour, and are ready to give out moisture whenever they are forced upwards or in any other way have their temperature lowered. The entire cquatorial zone becomes thus charged with vapour-laden air which is the primary necessity of a luxuriant vegetation. The surplus air (produced by the meeting of the two trade winds) which is ever rising in the equatorial
belt and giving up its store of vapour, flows off north and south as dry, cool air, and descends to the earth in the vicinity of the tropics. Here it sucks up whatever moisture it meets with and thus tends to keep this zone in an arid condition. The trades themselves are believed to be supplied by descending currents from the temperate zones, and these are at first equally dry and only become vapour-laden when they have passed over some extent of moist surface. At the solstices the sun passes vertically over the vicinity of the tropics for several weeks, and this further aggravates the aridity; and wherever the soil is sandy and there are no lofty mountain-chains to supply ample irrigation the result is a more or less perfect desert. Analogous causes, which a study of aerial currents will render intelligible, have produced other great forest-belts in the northern and southern parts of the temperate zones; but owing to the paucity of land in the southern hemisphere these are best seen in North America and Northern Euro-Asia, where they form the great northern forests of deciduous trees and of Coniferæ. These being comparatively wellknown to us, will form the standard by a reference to which we shall endeavour to point out and render intelligible the distinctive characteristics of the equatorial forest vegetation.
General Features of the Equatorial Forests. It is not easy to fix upon the most distinctive features of these virgin forests, which nevertheless impress themselves upon the beholder as something quite unlike those of tempe. ratc lands, and as possessing a grandeur and sublimity altogether their own. Amid the countless modifications in detail which these forests present, we shall endeavour