"120 



THE FARMER'S MAGAZINE. 



often gi'eat disasters. In tbe same maaner we have seen 

 springs nearly dry, in consequence of violent stonns give 

 out all at once with impetuosity, and dry up soon after.* 

 Those last observations, we can easily conceive, ought to 

 caution us not lightly to embrace the common opinion, 

 which assumes that the felling of wood.s diminishes the 

 annual quantity of rain ; for it is not impossible that not 

 only this quantity has not varied, but it may prove that 

 the volurae of the stream-waters has remained the same, 

 in spite of the appearance of drought presented at certain 

 seasons of the year, by the rivers and springs. Perhaps 

 we shall find that the only difterence is, that the flow of 

 the same mass of waters becomes more irregular by the 

 eftect of cutting down the woods. For instance, if the 

 lower waters of the Rhone, during a part of the year, 

 were exactly compensated by a sufficient number of large 

 floods, it would result that, at present, that river still 

 bears to the JMediterranean the same volume of water it 

 formerly poured into it at a period anterior to the clear- 

 ing of the woods that has taken place near its sources, and 

 when probably its mean depth was not, as in our daysi 

 subject to such considerable variations. If such was the 

 ease, the forests would have still this advantage, that they 

 regulate, and in some manner economise, the flow of water. 

 If, in reality, this flow becomes less abundant in propor- 

 tion as the clearings become more extended, it ought to 

 follow that the rains also are less plentiful ; or rather that 

 the evaporation is greatly promoted by depriving a soil of 

 trees, so that it is no longer sheltered from the sun's rays and 

 the wind. These two causes operate in the same way, and 

 ought often to combine ; and, without seeking to estimate 

 separately what belong* to each, we must, in the first place> 

 examine whether it is well established that the streams di- 

 minish on the face of a country in which great clearings are 

 effected ; in a word, we must ascertain whether we have not 

 taken an appearance of the fact for the reality. Here in- 

 deed lies the question, and we must see if nature has not 

 provided an order of phenomena that may serve as a guide 

 to solve it. 



The lakes without outlets that we meet with, whether on 

 the plains or on different elevations of tlie chains of moun- 

 tains, appear to me eminently proper to enlighten tlie dis- 

 cussion. We ought, in fact, to consider them as natural 

 gauges, destined to render valuable, on a colossal scale, the 

 variations to which the streams which water a country are 

 liable. If tlie mass of tliese waters are subject to a varia- 

 tion of any kind whatever, it is evident that that variation 

 will be indicated by tlie average level of the lake, for the 

 reason which causes the level of a lake to vary at different 

 seasons of the year, according as it pnives dry or rainy. 

 Tims the mean level of a lake will be lowered if tlie annual 

 quantity of running water it receives diminishes. On tlie 

 other hand, it will rise if the supply is increased. In short, 

 this level will remain stationary if the volume of water that 

 i^ returned tn tlie lake proves without variation. In this 

 discussion I liave preferred taking tlie case of lakes without 

 issue, the reason for which it is easy to perceive, since it 

 involves changes of level frequently very small. In the 

 meantime, I have not overlooked what relates to those lakes 

 which discharge their waters by a channel, because I am 

 convinced their study leads to very exact results. Before 

 going into the subject, I must give some explanation, in 

 order to define what I understand by changes of level. 



Geologists know that everywhere on the surface of the 



* Observations communicated by M. Larivi^re, at a sitting of 

 the Society of Natural Srience. 



globe the level of the waters appcan to have undergone 

 considerable variations, whether we seek for them on the 

 sea shores or on the borders of lakes. The fact is universal, 

 and not doubted by any one ; but they are not so generally 

 agreed upon the reality of the phenomenon ; some, and 

 tliat the majority, pretend that in many cases the change of 

 level is but apparent, that the masses of water are not 

 lowered, but tliat the banks liave been elevated ; others, on 

 the contrary, perceive a real disappearance of the mass of 

 liquid, a true drying up ; and reasons are urged in favour 

 of both points of view. It is not my task to take a part, 

 for tlie moment, in the dispute which divides the geologists ; 

 I shall have no occasion to occupy myself with the shores 

 washf J by the ocean ; nor shall I more avail myself of the 

 great difftiences of level that have evidently taken place in 

 certain lakfs, in consequence of certain geological circum- 

 stances, the examination of which is not in the limits of 

 my subject. These displacements, frequently enormous, 

 appear in general to have been occasioned by violent catas- 

 troplies, which, wiih very few exceptions, are anterior to 

 historical periods. I shall make use only of the changes 

 of level noticed in the lakes by our predecessors or our con- 

 temporaries. In a word, I shall attacix value only to facts 

 that arc accomplished under the eyes of men, since it is 

 the influence of these works upon the meteorological state 

 of the atmosphere that I propose to appreciate. What I 

 have to relate has been particularly observed in America. 

 However, I shall endeavour to establish, that what I believe 

 to be true for America will be applicable to every other 

 continent. 



One of the most interesting portions of Venezuela is un- 

 doubtedly tlie Valley of Aragna. Situated at a short dis- 

 tance from the coast, endowed with a warm climate, and a 

 soil of unexampled fertility, it embraces all the productions 

 proper to the tropical regions with those of Europe. Wheat 

 succeeds on the high grounds of Victoria. Limited on the 

 north by tlie cliain of sea coast, on the south by a system of 

 mountains, which separate it from tli^ Llanos, the Valley of 

 Aiagua is inclosed on the east and west by a series of hills 

 which eompletfly shut it in. By this singular conformation 

 the rivers which fake their rise in its interior, having no 

 embouchures towards the ocean, form by their union the 

 fine Lake of Taeanigua or Valencia, which, according to 

 Humboldt, eixeeds in extent that of Neufchatel, and is 

 eh-vated 4-39 metres (about 1,410 feet) above the sea. Its 

 length is about ten leagues, its greatest breadth does not 

 exceed twoand-a half leagues. At the time when Hum- 

 boldt visited the Valley of Aragua, the inliabitants were 

 suffering from the gradual dryiug-up of the lake, wliich had 

 been in progress for thirty years. Indeed it is quite suffi- 

 cient to compare the descriptions given of it by historians 

 with its present condition, tcTacknowledge, after allowing 

 largely for exaggerations, that the waters have considerably 

 diminished : the facts speak loudly enough. 



Oviedo, in his history of the province of Venezuela (re- 

 published ill 172.')), who, at the end of the I.'Jth ceiitnry, 

 had so often gone over the Valley of Aragua, says positively 

 that Nueva Valencia was founded in 155.5 at half a league 

 from the Lake of Tacaragua. In 1800 Humboldt found 

 that city 5,2(30 metres (about a leaguej from the shore. 



The aspect of the ground exlubits in other respects new 

 proofs; many hillocks of the plain still retain the names, to 

 this day, of isles which they formerly bore with more pro- 

 priety when they were surrounded with water. The lands, 

 laid bare by the retreat of the lake, were transformed into 

 admirable fields] of cottoil tree, bananas^ and sugar cane. 



