TREATISE BY HEBB WEX. 191 



flowing of them in times of flood, in the course of the last 140 years, 

 were to continue to operate also in the fdtnre, it is evident fiom what 

 has taken place that in brooks, and streams, and liveis, the lowest 

 and the Tnean levels of these may be expected to be lowered stiH 

 further in the future. And the question forces itself upon every one 

 involuntarily, to what degree may this diminution in the quantity of 

 water thus delivered by the several streams and rivers be carried ? 



" In the principal rivers, the Danube, Rhine, Elbe, and Vistula> it 

 is scarcely to be supposed that the lowering of the lowest level of the 

 flow will go on tin it reach the very ground of the bed of the stream, 

 occasioning the temporary drying up of the river altc^ether : 

 seeing that the two first named rivers are in part fed by the melting 

 of the masses of ice and snow lying on the Alps ; while, further, it is 

 to be hoped that the occasion of the lowering of the water level will 

 not be carried beyond a certain limit ; and finally, because the highest 

 and the lowest level of the stream occur in many of these streams 

 and affluents at different times of the year from what they do in 

 others. 



" I^ however, we look on the other hand to the very considerable 

 lowering of the lowest and the jaean levels of the rivers mentioned in 

 the foregoing table, which has taken place within the comparatively 

 short period of 50 years, we are brought to the disturbing conclusion 

 that these five rivers, in the course of 100 or of 200 years, may have 

 their lowest and average levels so lowered that they will be no longer 

 navigable if we do not take measures to counteract the cause which 

 is continuously operating to effect the diminution of the water flow 

 in these rivers. 



" The brooks and streams, on the contrary, which drain only basins 

 of limited extent, may, by the continuous lowering of the water level ; 

 and by the diminution of the delivery in them, be converted into inter- 

 mittent torrents, which He dry during many months of the year, and 

 then, on a Ml of rain, suddenly fill and deliver immense bodies of 

 water. 



That the dread of this is not without foundation is proved by 

 numerous cases in which very laige rivers, which in previous cen- 

 turies, according to historical records, were navigable at aU seasons 

 of the year, have become now only wildbdcke and torrents, such as, 

 for example, are most of the torrents which characterise the southern 

 slopes of the Alps in Italy and in Carinthia. Many other streams 

 and rivers, which not many decades back were of fdU water, have 

 latterly, and within the memory of man, become torrential streams, 



