【英文读物】The Tropical World.docx

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1、【英文读物】The Tropical WorldPREFACE.The numerous alterations I have made in this new edition of the Tropical World, both by the attentive revision of its former contents, and the addition of new chapters descriptive of the chief characteristics of the various tropical races of man; as well as the care I

2、 have taken to condense as much information as possible within narrow limits, will, I hope, justify my assertion that I have done my best to please indulgent readers, and to merit the favourable verdict of severer critics.Dr. HARTWIG.Salon Villas, Ludwigsburg (Würtemburg):March 10, 1873. CHAPTE

3、R I.THE DIVERSITY OF CLIMATES WITHIN THE TROPICS. Causes by which it is producedAbundance and Distribution of Rain within the TropicsThe Trade WindsThe Belt of CalmsTropical RainsThe MonsoonsTornadosCyclonesTyphoonsStorms in the PacificDevastations caused by Hurricanes on Pitcairn Island and Raroton

4、ga.On surveying the various regions of the torrid zone, we find that Nature has made many wonderful provisions to mitigate the heat of the vertical sun, to endow the equatorial lands with an amazing variety of climate, and to extend the benefit of the warmth generated within the tropics to countries

5、 situated far beyond their bounds.Thus, while the greater part of the northern temperate zone is occupied by land, the floods of ocean roll over by far the greater portion of the equatorial regionsfor both torrid America and Africa appear as mere islands in a vast expanse of sea.The conversion of wa

6、ter, by evaporation, into a gaseous form is accompanied by the abstraction of heat from surrounding bodies, or, in popular language, by the production of cold; and thus over the surface of the ocean the rays of the sun have a tendency to check their own warming influence, and to impart2 a coolness t

7、o the atmosphere, the refreshing effects of which are felt wherever the sea wind blows. There can, therefore, be no doubt that, if the greater part of the tropical ocean were converted into land, the heat of the torrid zone would be far more intolerable than it is.The restless breezes and currents,

8、the perpetual migrations of the air and waters, perform a no less important part in cooling the equatorial and warming the temperate regions of the globe. Rarefied by the intense heat of a vertical sun, the equatorial air-stream ascends in perpendicular columns high above the surface of the earth, a

9、nd thence flows off towards the poles; while, to fill up the void, cold air-currents come rushing from the arctic and antarctic regions.If caloric were the sole agent on which the direction of these antagonistic air-currents depended, they would naturally flow to the north and south; but the rotatio

10、n of the earth gradually diverts them to the east and west, and thus the cold air-currents, or polar streams, ultimately change into the trade winds which regularly blow over the greater part of the tropical ocean from east to west, and materially contribute, by their refreshing coolness, to the hea

11、lth and comfort of the navigator whom they waft over the equatorial seas.While the polar air-currents, though gradually warming as they advance, thus mitigate the heat of the torrid zone, the opposite equatorial breezes, which reach our coasts as moist south-westerly or westerly winds, soften the co

12、ld of our winters, and clothe our fields with a lively verdure during the greater part of the year. How truly magnificent is this grand system of the winds, which, by the constant interchange of heat and cold which it produces, thus imparts to one zone the beneficial influence of another, and render

13、s both far more fit to be inhabited by civilized man. The Greek navigators rendered homage to Æolus, but they were far from having any idea of the admirable laws which govern the unstable, ever-fluctuating domains of the God of the Winds.The same unequal influence of solar warmth under the lin

14、e and at the poles, which sets the air in constant motion, also compels the waters of the ocean to perpetual migrations, and produces those wonderful marine currents which like the analogous atmospheric streams, furrow in opposite directions the3 bosom of the sea. Thanks to this salutary interchange

15、, the Gulf Stream, issuing from the Mexican Sea, and thence flowing to the north and east, conveys a portion of its original warmth as far as the west coast of Spitzbergen and Nowaja Semlja; while in the southern hemisphere we see the Peruvian stream impart the refrigerating influence of the antarct

16、ic waters to the eastern coast of South America.The geographical distribution of the land within the tropics likewise tends to counterbalance or to mitigate the excessive heat of a vertical sun; for a glance over the map shows us at once that it is mostly either insular or extending its narrow lengt

17、h between two oceans, thus multiplying the surface over which the sea is able to exert its influence. The Indian Archipelago, the peninsula of Malacca, the Antilles, and Central America, are all undoubtedly indebted to the waters which bathe their coasts for a more temperate climate than that which

18、they would have had if grouped together in one vast continent.The temperature of a country proportionally decreases with its elevation; and thus the high situation of many tropical lands moderates the effects of equatorial heat, and endows them with a climate similar to that of the temperate, or eve

19、n of the cold regions of the globe. The Andes and the Himalaya, the most stupendous mountain-chains of the world, raise their snow-clad summits either within the tropics or immediately beyond their verge, and must be considered as colossal refrigerators, ordained by Providence to counteract the effe

20、cts of the vertical sunbeams over a vast extent of land. In Western Tropical America, in Asia, and in Africa, we find immense countries rising like terraces thousands of feet above the level of the ocean, and reminding the European traveller of his distant northern home by their productions and thei

21、r cooler temperature. Thus, by means of a few simple physical and geological causes acting and reacting upon each other on a magnificent scale, Nature has bestowed a wonderful variety of climate upon the tropical regions, producing a no less wonderful diversity of plants and animals.But warmth alone

22、 is not sufficient to call forth a luxuriant vegetation: it can only exert its powers when combined with a sufficient degree of moisture; and it chiefly depends upon the presence or absence of water whether a tropical country4 appears as a naked waste or decked with the most gorgeous vegetation.As t

23、he evaporation of the tropical ocean is far more considerable than that of the sea in higher latitudes, the atmospherical precipitations (dew, rain) caused by the cooling of the air are far more abundant in the torrid zone than in the temperate regions of the earth. While the annual fall of rain wit

24、hin the tropics amounts, on an average, to about eight feet, it attains in Europe a height of only thirty inches; and under the clear equatorial sky the dew is often so abundant as to equal in its effects a moderate shower of rain.But this enormous mass of moisture is most unequally distributed; for

25、 while the greater part of the Sahara and the Peruvian sand-coast are constantly arid, and South Africa and North Australia suffer from long-continued droughts, we find other tropical countries refreshed by almost daily showers. The direction of the prevailing winds, the condensing powers of high mo

26、untains and of forests, the relative position of a country, the nature of its soil, are the chief causes which produce an abundance or want of rain, and consequently determine the fertility or barrenness of the land. Of these causes, the first-mentioned is by far the most general in its effectsso th

27、at a knowledge of the tropical winds is above all things necessary to give us an insight into the distribution of moisture over the equatorial world.I have already mentioned the trade winds, or cool reactionary currents called forth by the ascending equatorial air-stream; but it will now be necessar

28、y to submit them to a closer examination, and follow them in their circular course throughout the tropical regions. In the Northern Atlantic, their influence, varying with the season, extends to 22° N. lat. in winter, and 39° N. lat. in summer; while in the southern hemisphere they reach no

29、farther than 18° S. lat. in winter, and 28° or 30° S. lat. in summer.In the Pacific, their limits vary between 21° and 31° N. lat., and between 23° and 33° S. lat.; so that, on the whole, they have here a more southern position, owing, no doubt, to the vast extent of open

30、 sea; while in the Atlantic the influence of the neighbouring continents forces them to the north, and even causes the trade winds of the southern hemisphere to ascend5 beyond the equatorial line. Their character is that of a continual soft breezestrongest in the morning, remitting at noon, and agai

31、n increasing in the evening. In the neighbourhood of the coasts, except over very small islands, they become weaker, and generally cease to be felt at a distance of about fifteen or twenty miles from the sea, though, of course, at greater heights they continue their course uninterruptedly over the l

32、and.For obvious reasons the trade winds have been much more accurately investigated upon the ocean than on land, particularly in the Northern Atlantic, which is better known in its physical features than any other sea, as being a highway for numberless vessels to which the study of the winds is a ma

33、tter of the greatest importance; yet, in spite of so many disturbing influences, their course, even over the continents, has been ascertained by travellers. North-easterly winds almost constantly sweep over the Sahara; and in South Africa, Dr. Livingstone informs us that north-easterly and south-eas

34、terly winds blow over the whole continent between 12° and 6° S. lat., even as far as Angola, where they unite with the sea winds.In Brazil, the presence of the trade winds has been determined with still greater accuracy. Thus easterly breezes almost perpetually sweep over the boundless plain

35、s up to the slopes of the Andes, and even in Paraguay (25° S. lat.) a mild east wind constantly arises in summer after the setting of the sun.As the trade winds originate in the coldest, and thence pass onwards to the warmer regions, they are, of course, constantly absorbing moisture as they adv

36、ance over the seas. Saturated with vapours, they reach the islands and continents, where, meeting with various refrigerating influences (mountain-chains, forests, terrestrial radiation), their condensing vapours give rise to an abundance both of rain and dew. It is owing to their influence that in g

37、eneral, within the tropics, the eastern coasts, or the eastern slopes of the mountains, are better watered than the interior of the continents or lands with a western exposure.An example on the grandest scale is afforded to us by South America, where the Andes of Peru and Bolivia so effectually drai

38、n the prevailing east winds of their moisture, that while numberless rivulets, the feeders of the gigantic Marañon, clothe their eastern gorges with a perpetual verdure, their western slopes are almost constantly arid. Such is the influence of this6 colossal barrier in interrupting the course

39、 of the air-current, that the trade wind only begins to be felt again on the Pacific at a distance of one hundred or even one hundred and fifty miles from the shore.In South Africa, also, we find the eastern mountainous coast-lands covered with giant timberin striking contrast with the parched savan

40、nas 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, howev

41、er, 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 others influence on meeting from the north and the south-east, and by the attraction of the sun, which, when in t

42、he 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 oc

43、cupying 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 summ

44、er, 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 sk

45、y, 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

46、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 the7 north and south of the belt of calms, we find in both hemispheres a broad zone, characterise

47、d 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-defi

48、ned 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 con

49、tinuance, 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 equa