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of the year; the burning air of the desert, having a much greater capacity for vapour, dissipates instantly all these clouds, which break up, vanish, and disappoint the excited expectation of the traveller, who hoped for refreshing rains.
Do the moist winds encounter an elevated obstacle, a high chain of mountains, a plateau ? Forced to ascend their slopes high into the atmosphere, they find there a colder air, which condenses their vapours, and the rain flows down along the sides. The wind passes over to the other side of the chain ; it arrives dry and cold, deprived of all its moisture, without clouds. The same wind thus brings rain on one side, and fair weather on the other. This is what happens every day on the two sides of the Scandinavian mountains.
In the equatorial regions, where the course of temperatures and winds is regular, that of the rains is equally so; and instead of seasons of temperature, which are there unknown, the inhabitants draw the distinguishing line between the dry and the rainy season.
Whenever the trade wind blows with its wonted regularity, the sky preserves a constant serenity and a deep azure blue, especially when the Sun is in the opposite atmosphere; the air is dry, and the atmosphere cloudless; but in proportion as the Sun approaches the zenith, the trade wind grows irregular, the sky assumes a whitish tint, it becomes overcast, clouds appear, sudden showers, accompanied with fierce storms, ensue. They occur more and more frequently, and turn at length into floods of rain, inundating the earth with torrents of water. The air is at this time so damp that the inhabitants are in an incessant vapour bath; the heat is heavy and stifling, the body becomes dull and enervated ; this is the period of those endemical fevers that destroy so great a number of the settlers who have come from the temperate zones. But vegetation puts on a new freshness and vigour ; the desert itself becomes animated, and is overspread for a few months with enchanting verdure, which furnishes pasture to thousands of animals. Nevertheless, ere long, the Sun, in his annual progress, advances to pour down his vertical rays upon other places; the rains diminish, the atmosphere becomes once more serene, the trade wind resumes its regularity, and the heaven shuts its windows once again until the following season.
Such is the normal course of the tropical rains. They fall everywhere during the passage of the Sun through the zenith. The heat is then so violent that the ascending current neutralizes the horizontal trade wind. It hurries the vapours to the heights of the atmosphere and the upper limit of the trade wind, where they are condensed, and fall back in a deluge of rain. Now, as the Sun passes and repasses from one tropic to the other, it follows that there is in the most intermediate places a twofold rainy season, the two periods of rain being more or less closely connected in point of time.
The quantity of water that falls from the atmosphere in the tropical regions during some months is enormous, if we compare it with that which we are accustomed to see water the soil of our own countries. It has been calculated that, on the average, there fall annually in the tropics of the Old World 77 inches of water, and 115 in tropical America. The mean for the equatorial region would be 96 inches.
But the annual quantity of rain received in some localities, and under the influence of certain circumstances, is sometimes much more considerable. At Paramaribo, in Dutch Guyana, it falls to the amount of 229 inches of water, or 19 feet. At St. Louis de Maranhao, in Brazil, 276 inches have been received, or 23 feet; but the greatest quantity erer observed is that of Mahabaleshwar, in the western Ghauts, south of Bombay, at the height of 4200 feet; it rises to the enormous number of 302 inches. A layer of 25 feet of water would have been formed by the rain waters, if they had not gradually run off.
These results are the more astonishing, as all this water falls in the space of only a few months, and, so to speak, at
It has been seen to fall at Cayenne 21 inches in a single day. This is nearly as much as falls during the whole year
in the northern latitudes. This is the reason why, notwithstanding the abundance of the rains, the number of clear days is much more considerable than in our climates. Even during the rainy season the Sun shows itself nearly every day, and many days pass without a single drop of water falling from the atmosphere.
We may conceive the prodigious effect such violent showers must produce upon the rivers. Who does not now understand the secret of the overflowings of the Nile, once so mysterious, and which are due to the circumstance that the region of its sources receives the tropical rains ?
Floods attaining an elevation of forty feet and upwards are frequent at this season in the great rivers of South
America ; the Llanos of the Orinoco are changed into an inland sea ; the Amazon inundates the plains through which it flows to a vast distance; the Paraguay forms lagoons, which, like those of Xarayes, are more than three hundred miles in length, and ooze away during the dry season.
The quantity of water contained in the tropical atmosphere in the condition of transparent gas, is always considerable. It is in proportion to the heat, which, being always very great, augments its capacity to a very high degree. Even under the most serene sky, the air is still abundantly provided with it. It is this invisible water which, being absorbed by the plants and taken up by their large leaves, produces the vigorous vegetation, and causes the eternal verdure that fills us with astonishment, under a sky devoid of rain, and cloudless during more than half the year; while in our climates, from the failure of rain for a few weeks only, we see all verdure languish, and all the flowers perish for the lack of moisture.
The distribution of rains in the temperate regions offers a perfect contrast to that of the tropics. Here, throughout the whole year, the earth is watered by the rains of heaven, although sometimes irregularly; but these are variable, as are the winds and the temperature, and secondary circumstances have much influence on them.-GUYOT.
Our insight into the distribution of heat in the atmosphere has been rendered more clear since the attempt has been made to connect together by lines those places where the mean annual summer and winter temperatures have been ascertained by correct observations. The system of isothermal, isotheral, and isochimenal lines, which I first brought into use in 1817, may, perhaps, if it be gradually perfected by the united efforts of investigators, serve as one of the main foundations of comparative climatology.
The term climate, taken in its most general sense, indicates all the changes in the atmosphere which sensibly affect our organs, as temperature, humidity, variations in the barometrical pressure, the calm state of the air or the action of opposite winds, the amount of electric tension, the purity of the atmosphere, or its admixture with more or less noxious
gaseous exhalations, and, finally, the degree of ordinary transparency and clearness of the sky, which is not only important with respect to the increased radiation from the Earth, the organic development of plants, and the ripening of fruits, but also with reference to its influence on the feelings and mental condition of men.
If the surface of the Earth consisted of one and the same homogeneous fluid mass, or of strata of rock having the same colour, density, smoothness, and power of absorbing heat from the solar rays, and of radiating it in a similar manner through the atmosphere, the isothermal, isotheral, and isochimenal lines would all be parallel to the equator.
In this hypothetical condition of the Earth's surface the power of absorbing and emitting light and heat would everywhere be the same under the same latitudes. The mathematical consideration of climate, which does not exclude the supposition of the existence of currents of heat in the interior or in the external coast of the Earth, nor of the propagation of heat by atmospheric currents, proceeds from this mean, and, as it were, primitive condition. Whatever alters the capacity for absorption and radiation, at places lying under the same parallel of latitude, gives rise to inflections in the isothermal lines. The nature of these inflections, the angles at wbich the isothermal, isotheral, or isochimenal lines intersect the parallels of latitude, their convexity or concavity with respect to the pole of the same hemisphere, are dependent on causes which, more or less, modify the temperature under different degrees of latitude.
In enumerating the causes which produce disturbances in the form of the isothermal lines, I would distinguish between those which raise and those which lower the temperature. To the first class belong the proximity of a western coast in the temperate zone, the divided configuration of a continent into peninsulas, with deeply indented bays and inland seas; the aspect or the position of a portion of the land with reference either to a sea of ice spreading far into the polar circle, or to a mass of continental land of considerable extent, lying in the same meridian, either under the equator, or, at least, within a portion of the tropical zone; the prevalence of southerly or westerly winds on the western shore of a continent in the temperate northern zone; chains of mountains acting as protecting walls against winds coming from colder regions; the infrequency of swamps, which, in the spring and beginning of summer, long remain covered with ice, and the absence of woods in a dry sandy soil ; finally, the constant serenity of the sky in the summer months, and the vicinity of an oceanic current, bringing water which is of a higher temperature than that of the surrounding sea.
Among the causes which tend to lower the mean annual temperature I include the following :-elevation above the level of the sea, when not forming part of an extended plain ; the vicinity of an eastern coast in high and middle latitudes; the compact configuration of a continent having no littoral curvatures or bays; the extension of land towards the poles into the region of perpetual ice, without the intervention of a sea remaining open in the winter; a geographical position in which the equatorial and tropical regions are occupied by the sea, and, consequently, the absence, under the same meridian, of a continental tropical land having a strong capacity for the absorption and radiation of heat; mountain-chains, whose mural form and direction impede the access of warm winds; the vicinity of isolated peaks occasioning the descent of cold currents of air down their declivities; extensive woods which hinder the insolation of the soil by the vital activity of their foliage, which produces great evaporation, owing to the extension of these organs, and increases the surface that is cooled by radiation, acting consequently in a threefold manner, by shade, evaporation, and radiation; the frequency of swamps or marshes, which in the north form a kind of subterranean glacier in the plains, lasting till the middle of the summer; a cloudy summer sky which weakens the action of the solar rays; and, finally, a very clear winter sky favouring the radiation of heat.
The simultaneous action of these disturbing causes, whether productive of an increase or decrease of heat, determines, as the total effect, the inflection of the isothermal lines, especially with relation to the expansion and configuration of solid continental masses, as compared with the liquid oceanic. These perturbations give rise to convex and concave summits of the isothermal curves. There are, however, different orders of disturbing causes, and each one must, therefore, be considered separately, in order that their total effect may afterwards be investigated with reference to the motion (direction, local curvature) of the isothermal lines, and the actions by which they are connected together, modified, destroyed, or increased in intensity, as manifested in the con