498 DIVERSION OF RIVERS. 
though on a much smaller scale, in Europe. The Torned and the 
Calix rivers in Lapland communicate by the Tarando, and in 
Westphalia, the Else, an arm of the Haase, falls into the Weser.* 
The change of bed in rivers by gradual erosion of their 
banks is familiar to all, but instances of the sudden abandon- 
ment of a primitive channel are by no means wanting. Ata 
period of unknown antiquity, the Ardéche pierced a tunnel 200 
feet wide and 100 high, through a rock, and sent its whole cur- 
rent through it, deserting its former bed, which gradually filled 
up, though its course remained traceable. In the great innun- 
dation of 1827, the tunnel proved insufficient for the discharge 
of the water, and the river burst through the obstructions which 
had now choked up its ancient channel, and resumed its original 
course.t 
It was probably such facts as these that suggested to ancient 
engineers the possibility of like artificial operations, and there 
are numerous instances of the execution of works for this pur- 
pose in very remote ages. The Bahr Jusef, the great stream 
which supplies the Fayoum with water from the Nile, has been 
supposed, by some writers, to be a natural channel; but both it 
and the Bahr el Wady are almost certainly artificial canals con- 
* The division of the currents of rivers, as a means of preventing the over- 
flow of their banks, is by no means a remedy capable of general application, 
even when local conditions are favorable to the construction of an emissary. 
The velocity of a stream, and consequently its delivery in a given time, are 
frequently diminished in proportion to the diminution of the volume by 
diversion ; and on the other hand, the increase of volume by the admission of 
a new tributary increases proportionally the velocity and the quantity of water 
delivered. Emissaries may, nevertheless, often be useful in carrying off water 
which has already escaped from the channel and which would otherwise 
become stagnant and prevent further lateral discharge from the main current, 
and it is upon this principle that Humphreys and Abbot think a canal of diver- 
sion at Lake Providence might be advisabie. Emissaries serve an important 
purpose in the lower course of rivers where the bed is nearly a dead level and 
the water moves from previously acquired momentum and the pressure of the 
current above, rather than by the force of gravitation, and it is, in general, 
only under such circumstances, as for example in the deltas at the mouths of 
great rivers, that nature employs them. 
7 Marpvieny, Mémoire sur les Inondations de 0 Ardéche, p. 13. 
