In treating the long-discussed question of the diminution of the waters, I conceive we must distinguish between the different periods at which the sinking of their level has taken place. Wherever we examine the valleys of rivers, or the basins of lakes, we see the ancient shore at great distances. No doubt seems now to be entertained, that our rivers and lakes have undergone immense diminutions; but many geological facts remind us also, that these great changes in the distribution of the waters have preceded all historical times; and that for many thousand years most lakes have attained a permanent equilibrium between the produce of the water flowing in, and that of evaporation and filtration. Whenever we find this equilibrium broken, it will be well rather to examine whether the rupture be not owing to causes merely local, and of very recent date, than to admit an uninterrupted diminution of the water. This reasoning is conformable to the more circumspect method of modern science. At a time when the physical history of the world, traced by the genius of some eloquent writers, borrowed all its charms from the fictions of imagination, the phenomenon of which we are treating would have been adduced as a new proof of the contrast these writers sought to establish between the two continents. To demonstrate that America rose later than Asia and Europe from the bosom of the waters, the lake of Tacarigua would have been described as one of those interior basins which have not yet become dry by the effects of slow and gradual evaporation. I have no doubt that, in very remote times, the whole valley, from the foot of the mountains of Cocuyza to those of Torito and Nirgua, and from La Sierra de Mariara to the chain of Guigue, of Guacimo, and La Palma, was filled with water. Everywhere the form of the promontories, and their steep declivities, seem to indicate the shore of an alpine lake, similar to those of Styria and Tyrol. The same little helicites, the same valvatae, which now live in the lake of Valencia, are found in layers of three or four feet thick as far inland as Turmero and La Concesion near La Victoria. These facts undoubtedly prove a retreat of the waters; but nothing indicates that this retreat has continued from a very remote period to our days. The valleys of Aragua are among the portions of Venezuela most anciently peopled; and yet there is no mention in Oviedo, or any other old chronicler, of a sensible diminution of the lake. Must we suppose, that this phenomenon escaped their observation, at a time when the Indians far exceeded the white population, and when the banks of the lake were less inhabited? Within half a century, and particularly within these thirty years, the natural desiccation of this great basin has excited general attention. We find vast tracts of land which were formerly inundated, now dry, and already cultivated with plantains, sugar-canes, or cotton. Wherever a hut is erected on the bank of the lake, we see the shore receding from year to year. We discover islands, which, in consequence of the retreat of the waters, are just beginning to be joined to the continent, as for instance the rocky island of Culebra, in the direction of Guigue; other islands already form promontories, as the Morro, between Guigue and Nueva Valencia, and La Cabrera, south-east of Mariara; others again are now rising in the islands themselves like scattered hills. Among these last, so easily recognised at a distance, some are only a quarter of a mile, others a league from the present shore. I may cite as the most remarkable three granite islands, thirty or forty toises high, on the road from the Hacienda de Cura to Aguas Calientes; and at the western extremity of the lake, the Serrito de Don Pedro, Islote, and Caratapona. On visiting two islands entirely surrounded by water, we found in the midst of brushwood, on small flats (four, six, and even eight toises height above the surface of the lake,) fine sand mixed with helicites, anciently deposited by the waters. (Isla de Cura and Cabo Blanco. The promontory of Cabrera has been connected with the shore ever since the year 1750 or 1760 by a little valley, which bears the name of Portachuelo.) In each of these islands may be perceived the most certain traces of the gradual sinking of the waters. But still farther (and this accident is regarded by the inhabitants as a marvellous phenomenon) in 1796 three new islands appeared to the east of the island Caiguira, in the same direction as the islands Burro, Otama, and Zorro. These new islands, called by the people Los nuevos Penones, or Los Aparecidos,* (* Los Nuevos Penones, the New Rocks. Los Aparecidos, the Unexpectedly-appeared.) form a kind of banks with surfaces quite flat. They rose, in 1800, more than a foot above the mean level of the water.
It has already been observed that the lake of Valencia, like the lakes of the valley of Mexico, forms the centre of a little system of rivers, none of which have any communication with the ocean. These rivers, most of which deserve only the name of torrents, or brooks,* are twelve or fourteen in number. (* The following are their names: Rios de Aragua, Turmero, Maracay, Tapatapa, Agnes Calientes, Mariara, Cura, Guacara, Guataparo, Valencia, Cano Grande de Cambury, etc.) The inhabitants, little acquainted with the effects of evaporation, have long imagined that the lake has a subterranean outlet, by which a quantity of water runs out equal to that which flows in by the rivers. Some suppose that this outlet communicates with grottos, supposed to be at great depth; others believe that the water flows through an oblique channel into the basin of the ocean. These bold hypotheses on the communication between two neighbouring basins have presented themselves in every zone to the imagination of the ignorant, as well as to that of the learned; for the latter, without confessing it, sometimes repeat popular opinions in scientific language. We hear of subterranean gulfs and outlets in the New World, as on the shores of the Caspian sea, though the lake of Tacarigua is two hundred and twenty-two toises higher, and the Caspian sea fifty-four toises lower, than the sea; and though it is well known, that fluids find the same level, when they communicate by a lateral channel.
The changes which the destruction of forests, the clearing of plains, and the cultivation of indigo, have produced within half a century in the quantity of water flowing in on the one hand, and on the other the evaporation of the soil, and the dryness of the atmosphere, present causes sufficiently powerful to explain the progressive diminution of the lake of Valencia. I cannot concur in the opinion of M. Depons* (who visited these countries since I was there) "that to set the mind at rest, and for the honour of science," a subterranean issue must be admitted. (* In his Voyage a la Terre Ferme M. Depons says, "The small extent of the surface of the lake renders impossible the supposition that evaporation alone, however considerable within the tropics, could remove as much water as the rivers furnish." In the sequel, the author himself seems to abandon what he terms "this occult case, the hypothesis of an aperture.") By felling the trees which cover the tops and the sides of mountains, men in every climate prepare at once two calamities for future generations; want of fuel and scarcity of water. Trees, by the nature of their perspiration, and the radiation from their leaves in a sky without clouds, surround themselves with an atmosphere constantly cold and misty. They affect the copiousness of springs, not, as was long believed, by a peculiar attraction for the vapours diffused through the air, but because, by sheltering the soil from the direct action of the sun, they diminish the evaporation of water produced by rain. When forests are destroyed, as they are everywhere in America by the European planters, with imprudent precipitancy, the springs are entirely dried up, or become less abundant. The beds of the rivers, remaining dry during a part of the year, are converted into torrents whenever great rains fall on the heights. As the sward and moss disappear with the brushwood from the sides of the mountains, the waters falling in rain are no longer impeded in their course; and instead of slowly augmenting the level of the rivers by progressive filtrations, they furrow, during heavy showers, the sides of the hills, bearing down the loosened soil, and forming sudden and destructive inundations. Hence it results, that the clearing of forests, the want of permanent springs, and the existence of torrents, are three phenomena closely connected together. Countries situated in opposite hemispheres, as, for example, Lombardy bordered by the Alps, and Lower Peru inclosed between the Pacific and the Cordillera of the Andes, afford striking proofs of the justness of this assertion.
Till the middle of the last century, the mountains round the valleys of Aragua were covered with forests. Great trees of the families of mimosa, ceiba, and the fig-tree, shaded and spread coolness along the banks of the lake. The plain, then thinly inhabited, was filled with brushwood, interspersed with trunks of scattered trees and parasite plants, enveloped with a thick sward, less capable of emitting radiant caloric than the soil that is cultivated and consequently not sheltered from the rays of the sun. With the destruction of the trees, and the increase of the cultivation of sugar, indigo, and cotton, the springs, and all the natural supplies of the lake of Valencia, have diminished from year to year. It is difficult to form a just idea of the enormous quantity of evaporation which takes place under the torrid zone, in a valley surrounded with steep declivities, where a regular breeze and descending currents of air are felt towards evening, and the bottom of which is flat, and looks as if levelled by the waters. It has been remarked, that the heat which prevails throughout the year at Cura, Guacara, Nueva Valencia, and on the borders of the lake, is the same as that felt at midsummer in Naples and Sicily. The mean annual temperature of the valleys of Aragua is nearly 25.5 degrees; my hygrometrical observations of the month of February, taking the mean of day and night, gave 71.4 degrees of the hair hygrometer. As the words great drought and great humidity have no determinate signification, and air that would be called very dry in the lower regions of the tropics would be regarded as humid in Europe, we can judge of these relations between climates only by comparing spots situated in the same zone. Now at Cumana, where it sometimes does not rain during a whole year, and where I had the means of collecting a great number of hygrometric observations made at different hours of the day and night, the mean humidity of the air is 86 degrees; corresponding to the mean temperature of 27.7 degrees. Taking into account the influence of the rainy months, that is to say, estimating the difference observed in other parts of South America between the mean humidity of the dry months and that of the whole year; an annual mean humidity is obtained, for the valleys of Aragua, at farthest of 74 degrees, the temperature being 25.5 degrees. In this air, so hot, and at the same time so little humid, the quantity of water evaporated is enormous. The theory of Dalton estimates, under the conditions just stated, for the thickness of the sheet of water evaporated in an hour's time, 0.36 mill., or 3.8 lines in twenty-four hours. Assuming for the temperate zone, for instance at Paris, the mean temperature to be 10.6 degrees, and the mean humidity 82 degrees, we find, according to the same formulae, 0.10 mill., an hour, and 1 line for twenty-four hours. If we prefer substituting for the uncertainty of these theoretical deductions the direct results of observation, we may recollect that in Paris, and at Montmorency, the mean annual evaporation was found by Sedileau and Cotte, to be from 32 in. 1 line to 38 in. 4 lines. Two able engineers in the south of France, Messrs. Clausade and Pin, found, that in subtracting the effects of filtrations, the waters of the canal of Languedoc, and the basin of Saint Ferreol lose every year from 0.758 met. to 0.812 met., or from 336 to 360 lines. M. de Prony found nearly similar results in the Pontine marshes. The whole of these experiments, made in the latitudes of 41 and 49 degrees, and at 10.5 and 16 degrees of mean temperature, indicate a mean evaporation of one line, or one and three-tenths a day. In the torrid zone, in the West India Islands for instance, the effect of evaporation is three times as much, according to Le Gaux, and double according to Cassan. At Cumana, in a place where the atmosphere is far more loaded with humidity than in the valley of Aragua, I have often seen evaporate during twelve hours, in the sun, 8.8 mill., in the shade 3.4 mill.; and I believe, that the annual produce of evaporation in the rivers near Cumana is not less than one hundred and thirty inches. Experiments of this kind are extremely delicate, but what I have stated will suffice to demonstrate how great must be the quantity of vapour that rises from the lake of Valencia, and from the surrounding country, the waters of which flow into the lake. I shall have occasion elsewhere to resume this subject; for, in a work which displays the great laws of nature in different zones, we must endeavour to solve the problem of the mean tension of the vapours contained in the atmosphere in different latitudes, and at different heights above the surface of the ocean.
A great number of local circumstances cause the produce of evaporation to vary; it changes in proportion as more or less shade covers the basin of the waters, with their state of motion or repose, with their depth, and the nature and colour of their bottom; but in general evaporation depends only on three circumstances, the temperature, the tension of the vapours contained in the atmosphere, and the resistance which the air, more or less dense, more or less agitated, opposes to the diffusion of vapour. The quantity of water that evaporates in a given spot, everything else being equal, is proportionate to the difference between the quantity of vapour which the ambient air can contain when saturated, and the quantity which it actually contains. Hence it follows that the evaporation is not so great in the torrid zone as might be expected from the enormous augmentation of temperature; because, in those ardent climates, the air is habitually very humid.
Since the increase of agricultural industry in the valleys of Aragua, the little rivers which run into the lake of Valencia can no longer be regarded as positive supplies during the six months succeeding December. They remain dried up in the lower part of their course, because the planters of indigo, coffee, and sugar-canes, have made frequent drainings (azequias), in order to water the ground by trenches. We may observe also, that a pretty considerable river, the Rio Pao, which rises at the entrance of the Llanos, at the foot of the range of hills called La Galera, heretofore mingled its waters with those of the lake, by uniting with the Cano de Cambury, on the road from the town of Nueva Valencia to Guigue. The course of this river was from south to north. At the end of the seventeenth century, the proprietor of a neighbouring plantation dug at the back of the hill a new bed for the Rio Pao. He turned the river; and, after having employed part of the water for the irrigation of his fields, he caused the rest to flow at a venture southward, following the declivity of the Llanos. In this new southern direction the Rio Pao, mingled with three other rivers, the Tinaco, the Guanarito, and the Chilua, falls into the Portuguesa, which is a branch of the Apure. It is a remarkable phenomenon, that by a particular position of the ground, and the lowering of the ridge of division to south-west, the Rio Pao separates itself from the little system of interior rivers to which it originally belonged, and for a century past has communicated, through the channel of the Apure and the Orinoco, with the ocean. What has been here effected on a small scale by the hand of man, nature often performs, either by progressively elevating the level of the soil, or by those falls of the ground occasioned by violent earthquakes. It is probable, that in the lapse of ages, several rivers of Soudan, and of New Holland, which are now lost in the sands, or in inland basins, will open for themselves a course to the shores of the ocean. We cannot at least doubt, that in both continents there are systems of interior rivers, which may be considered as not entirely developed; and which communicate with each other, either in the time of great risings, or by permanent bifurcations.
The Rio Pao has scooped itself out a bed so deep and broad, that in the season of rains, when the Cano Grande de Cambury inundates all the land to the north-west of Guigue, the waters of this Cano, and those of the lake of Valencia, flow back into the Rio Pao itself; so that this river, instead of adding water to the lake, tends rather to carry it away. We see something similar in North America, where geographers have represented on their maps an imaginary chain of mountains, between the great lakes of Canada and the country of the Miamis. At the time of floods, the waters flowing into the lakes communicate with those which run into the Mississippi; and it is practicable to proceed by boats from the sources of the river St. Mary to the Wabash, as well as from the Chicago to the Illinois. These analogous facts appear to me well worthy of the attention of hydrographers.
The land that surrounds the lake of Valencia being entirely flat and even, a diminution of a few inches in the level of the water exposes to view a vast extent of ground covered with fertile mud and organic remains.* (* This I observed daily in the Lake of Mexico.) In proportion as the lake retires, cultivation advances towards the new shore. These natural desiccations, so important to agriculture, have been considerable during the last ten years, in which America has suffered from great droughts. Instead of marking the sinuosities of the present banks of the lake, I have advised the rich landholders in these countries to fix columns of granite in the basin itself, in order to observe from year to year the mean height of the waters. The Marquis del Toro has undertaken to put this design into execution, employing the fine granite of the Sierra de Mariara, and establishing limnometers, on a bottom of gneiss rock, so common in the lake of Valencia.
It is impossible to anticipate the limits, more or less narrow, to which this basin of water will one day be confined, when an equilibrium between the streams flowing in and the produce of evaporation and filtration, shall be completely established. The idea very generally spread, that the lake will soon entirely disappear, seems to me chimerical. If in consequence of great earthquakes, or other causes equally mysterious, ten very humid years should succeed to long droughts; if the mountains should again become clothed with forests, and great trees overshadow the shore and the plains of Aragua, we should more probably see the volume of the waters augment, and menace that beautiful cultivation which now trenches on the basin of the lake.
While some of the cultivators of the valleys of Aragua fear the total disappearance of the lake, and others its return to the banks it has deserted, we hear the question gravely discussed at Caracas, whether it would not be advisable, in order to give greater extent to agriculture, to conduct the waters of the lake into the Llanos, by digging a canal towards the Rio Pao. The possibility* of this enterprise cannot be denied, particularly by having recourse to tunnels, or subterranean canals. (The dividing ridge, namely, that which divides the waters between the valleys of Aragua and the Llanos, lowers so much towards the west of Guigue, as we have already observed, that there are ravines which conduct the waters of the Cano de Cambury, the Rio Valencia, and the Guataparo, in the time of floods, to the Rio Pao; but it would be easier to open a navigable canal from the lake of Valencia to the Orinoco, by the Pao, the Portuguesa, and the Apure, than to dig a draining canal level with the bottom of the lake. This bottom, according to the sounding, and my barometric measurements, is 40 toises less than 222, or 182 above the surface of the ocean. On the road from Guigue to the Llanos, by the table-land of La Villa de Cura, I found, to the south of the dividing ridge, and on its southern declivity, no point of level corresponding to the 182 toises, except near San Juan. The absolute height of this village is 194 toises. But, I repeat that, farther towards the west, in the country between the Cano de Cambury and the sources of the Rio Pao, which I was not able to visit, the point of level of the bottom of the lake is much further north.) The progressive retreat of the waters has given birth to the beautiful and luxuriant plains of Maracay, Cura, Mocundo, Guigue, and Santa Cruz del Escoval, planted with tobacco, sugar-canes, coffee, indigo, and cacao; but how can it be doubted for a moment that the lake alone spreads fertility over this country? If deprived of the enormous mass of vapour which the surface of the waters sends forth daily into the atmosphere, the valleys of Aragua would become as dry and barren as the surrounding mountains.