TROPICAL NATURE, AND OTHER ESSAYS. I. THE CLIMATE AND PHYSICAL ASPECTS OF THE EQUATORIAL ZONE. The three Climatal Zones of the Earth-Temperature of the Equatorial Zone -Causes of the Uniform High Temperature near the Equator-Influence of the Heat of the Soil-Influence of the Aqueous Vapour of the Atmosphere-Influence of Winds on the Temperature of the Equator-Heat due to the Condensation of Atmospheric Vapour-General features of the Equatorial Climate-Uniformity of the Equatorial Climate in all parts of the globe-Effects of Vegetation on Climate-Short Twilight of the Equatorial Zone-The aspect of the Equatorial Heavens-Intensity of meteorological phenomena at the Equator-Concluding Remarks. IT is difficult for an inhabitant of our temperate land to realize either the sudden and violent contrasts of the arctic seasons or the wonderful uniformity of the equatorial climate. The lengthening or the shortening days, the ever-changing tints of spring, summer, and autumn, succeeded by the leafless boughs of winter, are constantly recurring phenomena which represent to us the estab lished course of nature. At the equator none of these changes occur; there is a perpetual equinox and 73 B a perpetual summer, and were it not for variations in the quantity of rain, in the direction and strength of the winds, and in the amount of sunshine, accompanied by corresponding slight changes in the development of vegetable and animal life, the monotony of nature would be extreme. In the present chapter it is proposed to describe the chief peculiarities which distinguish the equatorial from the temperate climate, and to explain the causes of the difference between them,-causes which are by no means of so simple a nature as are usually imagined. The three great divisions of the earth-the tropical, the temperate, and the frigid zones, may be briefly defined as the regions of uniform, of variable, and of extreme physical conditions respectively. They are primarily determined by the circumstance of the earth's axis not being perpendicular to the plane in which it moves round the sun; whence it follows that during one half of its revolution the north pole, and during the other half the south pole, is turned at a considerable angle towards the source of light and heat. This inclination of the axis on which the earth rotates is usually defined by the inclination of the equator to the plane of the orbit, termed the obliquity of the ecliptic. The amount of this obliquity is 23 degrees, and this measures the extent on each side of the equator of what are called the tropics, because within these limits the sun becomes vertical at noon twice a year, and at the extreme limit once a year, while beyond this distance it is never vertical. It will be evident, however, from the nature of the case, that the two lines which mark the limits of the geographical "tropics" will not define any abrupt |