in diminishhtg the Quantity of Water in Rivers. 87 



<0r to its evaporation being promoted by the ground not being 

 covered with trees, and being thus deprived of shelter against 

 the sun's I'ays, and the wind. These two causes, which al- 

 ways produce a similar effect, must generally be combined. 

 Before, however, endeavouring to estimate their influence sepa- 

 rately, it wiU be proper satisfactorily to ascertain if it be irre- 

 fragably established that the water-courses of a country where 

 clearing has been extensively accomplished, are truly dimi- 

 nished ; and thus assure ourselves, that the mere appearance of 

 the phenomenon be not taken for its reality. And after all, this 

 is the important point of the inquiry ; for let it be once esta- 

 blished, that the clearing of a country reduces the quantity of 

 water in its running streams, the cause of this diminution is of 

 secondary importance. We must now, therefore, inquire if we 

 can find in nature a set of phenomena which will act as a cri- 

 terion in resolving this question. 



I regard lakes, whether met with in plains, or in different 

 stages of mountain chains, as highly qualified to throw light 

 upon this discussion. These may be considered as natural 

 gauges, calculated to assist in valuing, upon a great scale, the 

 variations which may take place in the quantities of water 

 which fertilize a country. If the volume of these waters under- 

 go any variation, more or less, it is manifest that this varia- 

 tion, whether of excess or diminution, wiU be indicated by 

 the mean level of a lake, inasmuch as its mean level varies 

 at different times of the year, according as the season is wet 

 or dry. Thus, the mean level of a lake will fall, if the an- 

 nual quantity of running water in the streams of the district 

 diminishes ; on the contrary, it will rise, if these streams become 

 more copious, and, finally, the level will remain stationary, if 

 the volume of water which runs into the lake experiences no 

 change. In the following discussion, I have preferred the use 

 of those observations only which relate to lakes which have no 

 outlet ; and the reason will be apparent, as we wish to ascer- 

 tain changes, which may not be very considerable. However, 

 I do not disregard such lakes as have an issue for their waters ; 

 for I am certain that the study of such will also lead to very 

 precise results. Ik-fore proceeding farther, I shall say a few 

 vvords on tlu- meaning I attach to the phrase, change of level. 



