Masterpieces of Science 



any point in the course of one of our larger 

 rivers. The jetty causes ,the water to sweep 

 away from its obstruction and to strike against 

 the opposite shore. The crowding against the 

 shore gives its current increased power; it will 

 wrest away the alluvium from the grasp of 

 the roots, and will then cut under the trees, 

 causing considerable areas of forests to be 

 precipitated into the waters and borne away 

 to the sea. From the .point of impact, the cur- 

 rent will again rebound in a manner which will 

 cause it, at a certain distance below, to strike 

 against the opposite bank, where it will again 

 make swift encroachment against the forest 

 protection. After this second assault, it will 

 swing across to a lower point on the shore against 

 which it first impinged, and so the oscillations 

 from side to side will be propagated down stream 

 it may be for a hundred miles or more. A single 

 jetty of this description, as it has been observed 

 in the rivers of India, will affect the oscillations of 

 the current for an indefinite distance downward 

 in its course. That which is accomplished by 

 artifice in an immediate manner is more slowly 

 brought about by natural causes. Each tribu- 

 tary stream which enters the main channel 

 commonly has a greater swiftness of current 

 than the larger stream into which it flows. It 

 therefore bears in a mass of pebbles and builds a 

 natural jetty or bar at its mouth, thus gradually 

 forcing the current of the larger stream against 

 the opposite side, creating a bar there. It is 

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