Personal Narrative of Travels to the Equinoctial Regions of America, During the Years 1799-1804, Vol.2 — Chapter 2.16, Part 1

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Title: Personal Narrative of Travels to the Equinoctial Regions of America, During the Year 1799-1804 — Volume 2 Author: Alexander von Humboldt Aimé Bonpland Translator: Thomasina Ross Release date: December 1, 2004 [eBook #7014] Most recently updated: June 4, 2012 Language: English Other information and formats: www.

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org/ebooks/7014 Credits: Produced by Sue Asscher *** START OF THE PROJECT GUTENBERG EBOOK PERSONAL NARRATIVE OF TRAVELS TO THE EQUINOCTIAL REGIONS OF AMERICA, DURING THE YEAR 1799-1804 — VOLUME 2 *** Produced by Sue Asscher BOHN'S SCIENTIFIC LIBRARY.

HUMBOLDT'S PERSONAL NARRATIVE

VOLUME 2.

PERSONAL NARRATIVE OF TRAVELS TO THE EQUINOCTIAL REGIONS OF AMERICA DURING THE YEARS 1799-1804

BY

ALEXANDER VON HUMBOLDT AND AIME BONPLAND.

TRANSLATED FROM THE FRENCH OF

ALEXANDER VON HUMBOLDT

AND EDITED BY

THOMASINA ROSS.

IN THREE VOLUMES

VOLUME 2.

LONDON.

GEORGE BELL & SONS.

1907. LONDON: PORTUGAL ST.

, LINCOLN'S INN.

CAMBRIDGE: DEIGHTON, BELL AND CO.

NEW YORK: THE MACMILLAN CO.

BOMBAY: A.

H.

WHEELER AND CO.

*** A tablon, equal to 1849 square toises, contains nearly an acre and one-fifth: a legal acre has 1344 square toises, and 1. 95 legal acre is equal to one hectare.

A torta weighs three quarters of a pound, and three tortas cost generally in the province of Caracas one silver rial, or one-eighth of a piastre.

It is sufficient to mention, that the cubic foot contains 2,985,984 cubic lines.

Foot (old measure of France) about five feet three inches English measure.

VOLUME 2.

CONTENTS.

CHAPTER 2.

16. LAKE OF TACARIGUA.

--HOT SPRINGS OF MARIARA.

--TOWN OF NUEVA VALENCIA

DEL REY.

--DESCENT TOWARDS THE COASTS OF PORTO CABELLO.

CHAPTER 2.

17. MOUNTAINS WHICH SEPARATE THE VALLEYS OF ARAGUA FROM THE LLANOS OF CARACAS.

--VILLA DE CURA.

--PARAPARA.

--LLANOS OR STEPPES.

--CALABOZO.

CHAPTER 2.

18. SAN FERNANDO DE APURE.

--INTERTWININGS AND BIFURCATIONS OF THE RIVERS

APURE AND ARAUCA.

--NAVIGATION ON THE RIO APURE.

CHAPTER 2.

19. JUNCTION OF THE APURE AND THE ORINOCO.

--MOUNTAINS OF

ENCARAMADA.

--URUANA.

--BARAGUAN.

--CARICHANA.

--MOUTH OF THE

META.

--ISLAND OF PANUMANA.

CHAPTER 2.

20. THE MOUTH OF THE RIO ANAVENI.

--PEAK OF UNIANA.

--MISSION OF

ATURES.

--CATARACT, OR RAUDAL OF MAPARA.

--ISLETS OF SURUPAMANA AND

UIRAPURI.

CHAPTER 2.

21. RAUDAL OF GARCITA.

--MAYPURES.

--CATARACTS OF QUITUNA.

--MOUTH OF THE

VICHADA AND THE ZAMA.

--ROCK OF ARICAGUA.

--SIQUITA.

CHAPTER 2.

22. SAN FERNANDO DE ATABAPO.

--SAN BALTHASAR.

--THE RIVERS TEMI AND

TUAMINI.

--JAVITA.

--PORTAGE FROM THE TUAMINI TO THE RIO NEGRO.

CHAPTER 2.

23. THE RIO NEGRO.

--BOUNDARIES OF BRAZIL.

--THE CASSIQUIARE.

--BIFURCATION

OF THE ORINOCO.

CHAPTER 2.

24. THE UPPER ORINOCO, FROM THE ESMERALDA TO THE CONFLUENCE OF THE GUAVIARE.

--SECOND PASSAGE ACROSS THE CATARACTS OF ATURES AND

MAYPURES.

--THE LOWER ORINOCO, BETWEEN THE MOUTH OF THE RIO APURE, AND

ANGOSTURA THE CAPITAL OF SPANISH GUIANA.

*** PERSONAL NARRATIVE OF A JOURNEY TO THE EQUINOCTIAL REGIONS OF THE NEW CONTINENT.

VOLUME 2.

CHAPTER 2.

16. LAKE OF TACARIGUA.

HOT SPRINGS OF MARIARA.

TOWN OF NUEVA VALENCIA DEL REY.

DESCENT TOWARDS THE COASTS OF PORTO CABELLO.

The valleys of Aragua form a narrow basin between granitic and calcareous mountains of unequal height.

On the north, they are separated by the Sierra Mariara from the sea-coast; and towards the south, the chain of Guacimo and Yusma serves them as a rampart against the heated air of the steppes.

Groups of hills, high enough to determine the course of the waters, close this basin on the east and west like transverse dykes.

We find these hills between the Tuy and La Victoria, as well as on the road from Valencia to Nirgua, and at the mountains of Torito.

* (* The lofty mountains of Los Teques, where the Tuy takes its source, may be looked upon as the eastern boundary of the valleys of Aragua.

The level of the ground continues, in fact, to rise from La Victoria to the Hacienda de Tuy; but the river Tuy, turning southward in the direction of the sierras of Guairaima and Tiara has found an issue on the east; and it is more natural to consider as the limits of the basin of Aragua a line drawn through the sources of the streams flowing into the lake of Valencia.

The charts and sections I have traced of the road from Caracas to Nueva Valencia, and from Porto Cabello to Villa de Cura, exhibit the whole of these geological relations.

) From this extraordinary configuration of the land, the little rivers of the valleys of Aragua form a peculiar system, and direct their course towards a basin closed on all sides.

These rivers do not bear their waters to the ocean; they are collected in a lake; and subject to the peculiar influence of evaporation, they lose themselves, if we may use the expression, in the atmosphere.

On the existence of rivers and lakes, the fertility of the soil and the produce of cultivation in these valleys depend.

The aspect of the spot, and the experience of half a century, have proved that the level of the waters is not invariable; the waste by evaporation, and the increase from the waters running into the lake, do not uninterruptedly balance each other.

The lake being elevated one thousand feet above the neighbouring steppes of Calabozo, and one thousand three hundred and thirty-two feet above the level of the ocean, it has been suspected that there are subterranean communications and filtrations.

The appearance of new islands, and the gradual retreat of the waters, have led to the belief that the lake may perhaps, in time, become entirely dry.

An assemblage of physical circumstances so remarkable was well fitted to fix my attention on those valleys where the wild beauty of nature is embellished by agricultural industry, and the arts of rising civilization.

The lake of Valencia, called Tacarigua by the Indians, exceeds in magnitude the lake of Neufchatel in Switzerland; but its general form has more resemblance to the lake of Geneva, which is nearly at the same height above the level of the sea.

As the slope of the ground in the valleys of Aragua tends towards the south and the west, that part of the basin still covered with water is the nearest to the southern chain of the mountains of Guigue, of Yusma, and of Guacimo, which stretch towards the high savannahs of Ocumare.

The opposite banks of the lake of Valencia display a singular contrast; those on the south are desert, and almost uninhabited, and a screen of high mountains gives them a gloomy and monotonous aspect.

The northern shore on the contrary, is cheerful, pastoral, and decked with the rich cultivation of the sugar-cane, coffee-tree, and cotton.

Paths bordered with cestrums, azedaracs, and other shrubs always in flower, cross the plain, and join the scattered farms.

Every house is surrounded by clumps of trees.

The ceiba with its large yellow flowers* (* Carnes tollendas, Bombax hibiscifolius.

) gives a peculiar character to the landscape, mingling its branches with those of the purple erythrina.

This mixture of vivid vegetable colours contrasts finely with the uniform tint of an unclouded sky.

In the season of drought, where the burning soil is covered with an undulating vapour, artificial irrigations preserve verdure and promote fertility.

Here and there the granite rock pierces through the cultivated ground.

Enormous stony masses rise abruptly in the midst of the valley.

Bare and forked, they nourish a few succulent plants, which prepare mould for future ages.

Often on the summit of these lonely hills may be seen a fig-tree or a clusia with fleshy leaves, which has fixed its roots in the rock, and towers over the landscape.

With their dead and withered branches, these trees look like signals erected on a steep cliff.

The form of these mounts unfolds the secret of their ancient origin; for when the whole of this valley was filled with water, and the waves beat at the foot of the peaks of Mariara (the Devil's Nook* (* El Rincon del Diablo.

)) and the chain of the coast, these rocky hills were shoals or islets.

These features of a rich landscape, these contrasts between the two banks of the lake of Valencia, often reminded me of the Pays de Vaud, where the soil, everywhere cultivated, and everywhere fertile, offers the husbandman, the shepherd, and the vine-dresser, the secure fruit of their labours, while, on the opposite side, Chablais presents only a mountainous and half-desert country.

In these distant climes surrounded by exotic productions, I loved to recall to mind the enchanting descriptions with which the aspect of the Leman lake and the rocks of La Meillerie inspired a great writer.

Now, while in the centre of civilized Europe, I endeavour in my turn to paint the scenes of the New World, I do not imagine I present the reader with clearer images, or more precise ideas, by comparing our landscapes with those of the equinoctial regions.

It cannot be too often repeated that nature, in every zone, whether wild or cultivated, smiling or majestic, has an individual character.

The impressions which she excites are infinitely varied, like the emotions produced by works of genius, according to the age in which they were conceived, and the diversity of language from which they in part derive their charm.

We must limit our comparisons merely to dimensions and external form.

We may institute a parallel between the colossal summit of Mont Blanc and the Himalaya Mountains; the cascades of the Pyrenees and those of the Cordilleras: but these comparisons, useful with respect to science, fail to convey an idea of the characteristics of nature in the temperate and torrid zones.

On the banks of a lake, in a vast forest, at the foot of summits covered with eternal snow, it is not the mere magnitude of the objects which excites our admiration.

That which speaks to the soul, which causes such profound and varied emotions, escapes our measurements as it does the forms of language.

Those who feel powerfully the charms of nature cannot venture on comparing one with another, scenes totally different in character.

But it is not alone the picturesque beauties of the lake of Valencia that have given celebrity to its banks.

This basin presents several other phenomena, and suggests questions, the solution of which is interesting alike to physical science and to the well-being of the inhabitants.

What are the causes of the diminution of the waters of the lake?

Is this diminution more rapid now than in former ages?

Can we presume that an equilibrium between the waters flowing in and the waters lost will be shortly re-established, or may we apprehend that the lake will entirely disappear?

According to astronomical observations made at La Victoria, Hacienda de Cura, Nueva Valencia, and Guigue, the length of the lake in its present state from Cagua to Guayos, is ten leagues, or twenty-eight thousand eight hundred toises.

Its breadth is very unequal.

If we judge from the latitudes of the mouth of the Rio Cura and the village of Guigue, it nowhere surpasses 2. 3 leagues, or six thousand five hundred toises; most commonly it is but four or five miles.

The dimensions, as deduced from my observations are much less than those hitherto adopted by the natives.

It might be thought that, to form a precise idea of the progressive diminution of the waters, it would be sufficient to compare the present dimensions of the lake with those attributed to it by ancient chroniclers; by Oviedo for instance, in his History of the Province of Venezuela, published about the year 1723. This writer in his emphatic style, assigns to "this inland sea, this monstruoso cuerpo de la laguna de Valencia"* (* "Enormous body of the lake of Valencia."

), fourteen leagues in length and six in breadth.

He affirms that at a small distance from the shore the lead finds no bottom; and that large floating islands cover the surface of the waters, which are constantly agitated by the winds.

No importance can be attached to estimates which, without being founded on any measurement, are expressed in leagues (leguas) reckoned in the colonies at three thousand, five thousand, and six thousand six hundred and fifty varas.

* (* Seamen being the first, and for a long time the only, persons who introduced into the Spanish colonies any precise ideas on the astronomical position and distances of places, the legua nautica of 6650 varas, or of 2854 toises (20 in a degree), was originally used in Mexico and throughout South America; but this legua nautica has been gradually reduced to one-half or one-third, on account of the slowness of travelling across steep mountains, or dry and burning plains.

The common people measure only time directly; and then, by arbitrary hypotheses, infer from the time the space of ground travelled over.

In the course of my geographical researches, I have had frequent opportunities of examining the real value of these leagues, by comparing the itinerary distances between points lying under the same meridian with the difference of latitudes.

) Oviedo, who must so often have passed over the valleys of Aragua, asserts that the town of Nueva Valencia del Rey was built in 1555, at the distance of half a league from the lake; and that the proportion between the length of the lake and its breadth, is as seven to three.

At present, the town of Valencia is separated from the lake by level ground of more than two thousand seven hundred toises (which Oviedo would no doubt have estimated as a space of a league and a half); and the length of the basin of the lake is to its breadth as 10 to 2. 3, or as 7 to 1. 6. The appearance of the soil between Valencia and Guigue, the little hills rising abruptly in the plain east of the Cano de Cambury, some of which (el Islote and la Isla de la Negra or Caratapona) have even preserved the name of islands, sufficiently prove that the waters have retired considerably since the time of Oviedo.

With respect to the change in the general form of the lake, it appears to me improbable that in the seventeenth century its breadth was nearly the half of its length.

The situation of the granite mountains of Mariara and of Guigue, the slope of the ground which rises more rapidly towards the north and south than towards the east and west, are alike repugnant to this supposition.

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.