48 POPULAR SCIENCE MONTHLY. 



it, which material is generally the product of previous erosions. 

 Whenever the current needs all its strength to carry the material it 

 has in suspension, together with the solid matter it is pushing along, 

 it will have lost all its power to cause erosion. If the slope is de- 

 creased, or if the matter in suspension is increased in any manner, 

 deposits will occur. These deposits render the slope less steep at that 

 point but steeper below, so that the action of the water beyond will 

 make itself felt, and by digging gradually up stream tend to restore 

 the original slope. 



In some cases there is very little erosion where the current is 

 much stronger than the cohesive power of the soil, for the reason 

 that the beds of the streams have been almost paved with stones that 

 have been carried along by the propulsive action of the water. 



This has in many cases produced an equilibrium between the 

 resistance and the destroying power; in others the equilibrium has 

 been brought about in a different way by the same natural agents. 

 The current of a stream will very often go on causing erosion until 

 arrested by some rocky obstacle that determines a waterfall. These 

 falls cause breaks in the action of the water not only in stopping the 

 erosive action in its upward march, but also in checking the velocity 

 of the water. Then, as basins are often formed just above the falls 

 and where the current is much less swift, matter in suspension is 

 deposited, so that when the stream is swollen it has material to 

 work upon, before starting to make the original slope steeper. 



The subject divides itself broadly into two branches — the extinc- 

 tion of torrents and the correction of the water courses in valleys. 

 In the extinction of the torrents various plans are resorted to, 

 which give the current greater propulsive power, but at the same time 

 they render necessary greater protection of the bed. This may be 

 done by incasing it within walls of masonry (though other materials 

 are used in some cases) or shortening meandering portions. In the 

 latter plan the slope is increased, the fall being the same for a shorter 

 distance. Currents that have been making dangerous deposits at cer- 

 tain points and causing dangerous erosions at others are treated by 

 the above systems until the danger has disappeared or the money 

 has given out. When the erosive action of the water is already too 

 great, the material carried and then deposited by the stream is often 

 made use of to consolidate banks that are threatened. Spurs are 

 built out from such banks, and this tends to mend matters not only 

 in forcing the water to take another channel, but also in causing 

 deposits at the foot of the menaced bank. 



The destructive effects of the current are arrested when the 

 streams are not important by means of dams made of trunks of trees 

 and wooden stakes, often strengthened roughly with stones. Where 



