Graham's Magazine, Vol. XLI, No. 6, December 1852. Various
of eight or ten minutes, to a negative state. The rainy season is that of thunder-storms. The storm rises in the plains two hours after the sun passes through the meridian, and therefore shortly after the period of the maximum of the diurnal heat in the tropics. In the inland districts it is exceedingly rare to hear thunder at night or in the morning – nocturnal thunder-storms being peculiar to certain valleys of rivers which have a particular climate.” The substance of the explanation of the preceding phenomena, by the philosophic writer of the statement, may be briefly given:
The season of rains and thunder in the northern equinoctial zone coincides with the passage of the sun through the zenith of the place, the cessation of the breezes or north-east winds, and the frequency of calms and furious currents of the atmosphere from the south-east and south-west, accompanied with a cloudy sky. While the breeze from the north-east blows, it prevents the atmosphere from being saturated with moisture. The hot and loaded air of the torrid zone rises, and flows off again toward the poles, while inferior currents from these last, bringing drier and colder strata, take the place of the ascending columns. In this manner, the humidity – being prevented from accumulating – passes off toward the temperate and colder regions, so that the sky is always clear. When the sun, entering the northern signs, rises toward the zenith, the breeze from the north-east softens, and at length ceases; this being the season at which the difference of temperature between the tropics and the contiguous zone is least. The column of air resting on the equinoctial zone becomes replete with vapors, because it is no longer renewed by the current from the pole: clouds form in this atmosphere, saturated and cooled by the effects of radiation and the dilatation of the ascending air, which increases its capacity for heat in proportion as it is rarefied. Electricity accumulates in the higher regions, in consequence of the formation of the vesicular vapors, the precipitation of which is constant during the day, but generally ceases at night. The showers are more violent and accompanied with electrical explosions, shortly after the maximum of the diurnal heat. These phenomena continue until the sun enters the southern signs, when the polar current is reëstablished, because the difference between the heat at the equinoctial and temperate regions is daily increasing. The air of the tropics being thus renewed, the rains cease, the vapors are dissolved, and the sky resumes its azure tint.
The Orinoco, when in flood, inundates a vast extent of country, six hundred miles in length, and from sixty to ninety in width. Its waters cover the savannas along its banks to the depth of twelve or fourteen feet, giving to them a lake-like appearance, in the midst of which farm-houses and villages are seen rising on islands but little elevated above the surface. The wild cattle perish in great numbers, and fall an easy prey to the carrion-vultures and alligators. In one part of the river Humboldt found marks of recent inundation at 45 feet above the ordinary level; but above the greatest height to which its waters are now elevated, he traced its ancient action at 106 or even 138 feet. “Is this river, then,” inquires he, “the Orinoco, which appears to us so imposing and majestic, merely the feeble remnant of those immense currents of fresh water which, swelled by Alpine snows or by more abundant rains, every where shaded by dense forests, and destitute of those beaches that favor evaporation, formerly traversed the regions to the east of the Andes, like arms of inland seas? What must then have been the state of those low countries of Guiana, which now experience the effects of annual inundations! What a prodigious number of crocodiles, lamartines, and boas must have inhabited these vast regions, alternately converted into pools of stagnant water and arid plains! The more peaceful world in which we live has succeeded to a tumultuous world. Bones of mastadons and real American elephants are found dispersed over the platforms of the Andes. The megatherium inhabited the plains of Uruguay. By digging the earth more deeply in high valleys, which at the present day are unable to nourish palms or tree-ferns, we discover strata of coal, containing gigantic remains of monocotyledonous plants. There was therefore a remote period when the tribes of vegetables were differently distributed, when the animals were larger, the rivers wider and deeper. There stop the monuments of nature which we can consult.”
The bifurcation of flowing waters is sufficiently illustrative of the physics of the earth to justify a few words: – Europe presents two instances of bifurcation – one in Italy, between the Arno and the Chiana; the other in Germany, between the Haase and the Else, in Westphalia. Asia also possesses, on the peninsula lying beyond India, two grand examples. What we know about them is principally founded upon the information gathered by Dr. Buchanan Hamilton, during his stay in Ava, the capital of the Birman empire. But it is to be observed, that the communications of these Indian rivers, at least as regards those in the country of the Jun-Shan, appear doubtful. British travelers have succeeded in penetrating, from Maulmain, at the mouth of the river Saluan, into the interior of the country of the Shan, which has been so long shut up; but Lieutenant M’Leod, who reached the river of Kambodja, says nothing to confirm the information Dr. Buchanan gives us. The most important of all known divisions in the form of a fork, however, is the bifurcation of the Orinoco, which communicates through the Cassiquiare with the Rio Negro, and through this river with the Amazon. It has already been remarked, that the observations of A. von Humboldt have put this bifurcation beyond a doubt; but the subject deserves a recurrence to it, as presenting to our attention a singular physical feature, and illustrating the energy of the great traveler of modern times.
He and Bonpland left Carracas in the year 1800, crossed the valleys of Aragua, and the Llanos of Calabozo – excellent pastures, which separate the cultivated part of Venezula from the region of the forests and missions – and embarked at San Fernando, on the Rio Apure, to follow its course downward to its discharge into the principal branch of Orinoco. They then ascended the Orinoco, passing its two great cataracts of Apures and Maypures, and reached the village of San Fernando de Atabapo, situated at the junction of the Guaviare and Atabapo, and near lat. 4° N. Here they left the river, and sailed up the Atabapo to the mouth of the Rio Temi, which latter they followed as far as its confluence with the Tuamini, and arrived at the village of San Antonio de Javita, formerly mentioned as remarkable for its amount of rain. From this point the Indians carried their boat across the isthmus which separates the Tuamini from the Rio Pimichin, the travelers following on foot, passing through dense forests, often in danger from the number of snakes that infested the marshes. Embarking on the Pimichin, they came in four hours and a half into the Rio Negro. “The morning,” says Humboldt, “was cool and beautiful. We had been confined thirty-six days in a narrow canoe, so unsteady that it would have been upset by any one rising imprudently from his seat, without warning the rowers to preserve its balance by leaning to the opposite side. We had suffered severely from the stings of insects, but had withstood the insalubrity of the climate; we had passed without accident the numerous falls and bars that impede the navigation of the rivers, and often render it more dangerous than long voyages by sea. After all we had endured, I may be allowed to mention the satisfaction which we felt in having reached the tributaries of the Amazon.” The Rio Negro, which flows into that river, was navigated downward as far as San Carlos, then supposed to lie under the equator, but actually about 2° N. From thence the travelers retraced the river, passed from it into the Cassiquiare, and again entered the main channel of the Orinoco, three leagues below the mission of Esmeralda; thus demonstrating a junction between the two great floods of the Amazon and Orinoco, which had been, in the year 1798, declared by Bauche to be a geographical monstrosity. The bifurcation of the Orinoco takes place in the following manner: – The river, issuing from among the mountains, reaches the opening of a valley or depression which terminates at the Rio Negro. Here it divides into two branches, the smaller, or the Cassiquiare, turning off to the south, while the main stream continues its original direction – west-north-west. A reference to Humboldt’s map, of which we give a translated copy, will render further explanation unnecessary.
The preceding notices refer to what have been appropriately styled the “might rivers,” and the “great rivers,” none of which are to be found in Europe. Its noblest running waters belong to a third grade. “These,” says Inglis, “I would designate the large rivers; for great and large are not entirely synonymous; and, to most minds, the term great river and large river, will present a distinct image. The lower we descend in the scale, the more numerous do we find the species. The continent of Europe abounds with examples of the third class – such as the Rhine, the Danube, the Rhone, the Elbe, the Tagus, the Ebro, and the Guadalquivir. The fourth class is still more numerous; and of this class, which I would call considerable rivers, we may find examples