THE FARMER'S MAGAZINE. 



267 



Saussure, ia his first researches iuto tho temperature of 

 the lakes, examiuetl those placed at the foot of the first 

 range of the Jura. The Lake of Neufchatel is eight leagues 

 in length, and the greatest width does not exceed two leagues. 

 Saussure was struck, in visitmg it, mth the extent that it 

 must formerly have had ; " for," says he, " the great hori- 

 zontal meadows and marshes which terminate it to the 

 south-west have indubitably been covered with water, as 

 have been the plaias that we crossed up to the Lake of 

 Bieuue." 



The Lake Slorat is also separated from the Lake of Neuf- 

 chatel by horizontal marshes, which beyond a doubt were 

 formerly submerged. " Then," adds Saussure, " the tln-ee 

 great lakes of Neufchatel, Bienne, and Morat were included 

 in one basin." 



In Switzerland, as ia America and Asia, the ancient lakes, 

 which#ve might call the primitive lakes, those which occu- 

 pied the bottoms of valleys when the country was wild and 

 uncultivated, have been divided, by the effect of drought, 

 into a certain number of independent lakes. 



I shall finish the task 1 have imposed upon myself, by 

 utilizing, for the benefit of the discussion, the observations 

 of Saussiu-e on the Lake of Geneva, That lake is, it may 

 be said, the point of setting out of the immense labours of 

 that celebrated physician. No one has studied it more 

 than he. 



Saussure admits that at a period much anterior to his- 

 toric times the mountains which overlook the Lake were 

 buried under tho waters. A catastrophe occasioned a break- 

 mg-up, and soon the current occupies only the base of the 

 valley, and the Lake of Geneva was formed. 



In basing our opinion on the monuments constructed by 

 men, we should have no doubt that for twelve or thirteen 

 hundred years the waters of the Lake of Geneva have been 

 gradually retiring. It is decidedly evident, that upon the 

 shores they have abandoned, the quarter of Kive, and the 

 low streets have been built.* " This lowering of the surface 

 of the level of the Lake," continues Saussure, "is not alone 

 the effect of the deepening of the channel of discharge ; it 

 has been produced also by a dimiiiution of the quantity of 

 water flowing into it. 



The consequence we are allowed to deduce from the 

 observations of Saussure is, that for twelve or thirteen hun- 

 dred years the water-courses have gradually Uiraiuished in 

 the counties adjacent to the Lake of Geneva. None I tliink 

 will dispute that during that long period there have been 

 in SwitzerJRnd immense clearings, and a continual progi-ess 

 in the cultivation of that fine country. By an examination 

 of the level of the lakes we have arrived at this conclusion, 

 that in countries in which have been carried on extensive 

 clearings there has probably been a diminution of the 

 running waters which flows over the surface of the land. 



Forests should therefore have the effect, in the fu-st 

 instance, of preservmg the waters, by fonning a barrier 

 to their uniting and flowing with too great a rapidity, 

 and by presenting an obstacle to their evaporation, 



That a soil covered with trees is less favourable to 

 evaporation than a country denuded of trees is what every 

 body wU admit without dispute. But in order the better 

 to observe the difference of these two conditions, it is 

 necessary after a rainy season to go over a road which 

 raverses successively an open and a wooded country. We 

 then remark that the forest road is still covered with mud, 

 whilst that in the open country is entirely dry. 



It is, above all, in South America that the diflSculty of 

 evaporation on a soil shaded by abundant vegetation is 

 most decided. In the forests there, the humidity is con- 

 stant, even a long time after the rainy season. The hills 

 remain during the whole year true bogs. The only means 

 of drying these forest roads is, by giving them a width of 

 80 to 100 metres, which is as much as to say that it is 

 necessary to make a real clearing. 



If it is once admitted that the streams are diminished 

 by the efiect of clearings and cultivation, it is proper to 

 examine whether this diminution proceeds from a less 

 quantity of rain, a greater evaporation, or rather to the ex- 

 haustion of irrigation. 



I have laid down the principle at the commencement of 

 this memoir that it is necessarily nearly impossible to 



• Saussure, Voyage dans les Alpes, t 2, eh. 16. 



assign precisely the pai-t of each of these several causes. 

 I will however endeavour to estimate them in a general 

 way ; and the discussion will gain something if I prove that 

 there may be a diminution of the streams from the sole 

 efiect of clearings, without taking into account those other 

 causes which act at the same time. 



In regard to irrigation 1 must necessarily make a distinc- 

 tion between the case in wliich high cultivation is substi- 

 tuted for the forest, and that in which an arid soil destitute 

 of wood is rendered cultivable by the industry of man. In 

 the first case, it is very probable that irrigation alleviates in 

 a very slight degree the body of the stream ; for we must 

 admit that the quantity of water consumed for supplying the 

 vegetation of a given surface of forest, equals, if it does not 

 surpass, that winch would be absorbed by a similar surface 

 appropriated to cultivation after the clearing. Then the 

 influence exercised by this cultivated land enters into the 

 condition of a cleared soil, acting especially by favouriag 

 evaporation. In the second case, that is, in that in which a 

 great extent of uncultivated country shall have been con- 

 verted by tillage, there would evidently be a great consump- 

 tion of water by the vegetation which would have been 

 promoted by it. The mtroduction therefore of agricultural 

 mdustry will tend to diminish the streams that flow through 

 such a country. It is very probably to such a circumstance 

 that we must attribute the gradual drying up of the lakes 

 in which are collected a great part of the running waters of 

 Northern Asia. 



In the considerations on the lakes of Venezuela, New 

 Grenada, and Switzerland, I said that we might attribute 

 the disappearance of a part of the streams, tributaries of 

 those lakes, to a less quantity of rain ; but it might be 

 sustamed with quite as much reason that it is simply the 

 consequence of a more rapid evaporation. It is in reality 

 from circumstances, under the influence of which the 

 diminution of the rumiing waters is occasioned by a more 

 active evaporation. We have quoted on this subject 

 numerous examples ; but in a discussion of this kind it is 

 only facts well authenticated that it is safe to accept. From 

 this motive I shall confine myself to stating two observa- 

 tions; one made in the Isle of Ascension by M. Debassyns 

 de Eichemont, the other in the Mines of Marmato. 



In the Isle of Ascension he saw a fine spring, situated 

 at the foot of a mountain originally wooded, lose its 

 abundance and drj'-up where the trees had been cut down. 

 It was planted again, and some years after the spring 

 re-appeared by degrees, and soon flowed with its accuS' 

 tomed abundance. Its temporary loss therefore was justly 

 ascribed to the cutting down of the timber. 



The metalliferic mountain of Marmato is situated in 

 the province of Popayau, in the midst of immense forests. 

 The stream upon winch the stamping mills are erected is 

 formed by the junction of several small rivulets, which take 

 their rise on tire plateau of Saint George. 



In 1826, when I visited these mines for the first time, 

 Marmato consisted of several miserable huts, inhabited by 

 negro slaves. In 1830, the period at which I finally left 

 that locality, Marmato presented a more animated aspect. 

 Large workshops had risen up, mih a gold foundry, and 

 machines for separating and amalgamating the minerals. 

 A free population of nearly 3,0C0 inhabitants found them- 

 selves gradually collected on the slope of the mountain. 

 For tliis pmpose a copious fall of timber had been effected, 

 as well for building purposes as for making charcoal. In 

 order to facilitate the U-ansport the cutting had been 

 extended to the plateau of San Jorge. The clearing had 

 scarcely proceeded for two years before it was perceived 

 that the volume of water employed in working the machines 

 had sensibly diminished. Its amount was ascertamed by 

 the working of the machines.* The question was a serious 

 one, for, at Marmato, a diminution in the quantity of 

 working water will always be followed by a diminution in 

 the quantity of gold produced. 



At Marmata and at the Isle of Ascension, it is not at 

 all probable that a clearing so local and limited would 

 exercise any influence on tlie meteorological state of the 

 atmosphere, so as to vary the quantity of rain-faU still 

 more; at Mannato, as soon as they had ascertairred the 



* A gauge taken exactly at different times has proved the real 

 diminution of the motive waters. 



