CHAPTER IX. 



THE WORK OF RAIN AND RIVERS. 



Take a gallon or two of water out of the Thames at 

 London Bridge, and allow it to stand quietly in a clean 

 vessel. If you look at it, after it has stood for several 

 hours, you will find that tlie water is much clearer, and 

 that a quantity of muddy matter is spread over the 

 bottom of the vessel, the quantity being greater or less 

 according to the condition of the river at the time you 

 happen to examine it. This mud was previously held in 

 suspension by the water, and was the main cause of its 

 turbidity, so that, as soon as the muddy particles settled, 

 the water became clearer. While the water was in the 

 river, the fine solid particles were kept in constant agita- 

 tion by the current of the stream, and were thus prevented 

 from settling dov/n. The more rapid the stream, the greater 

 is its power of carrying this suspended matter ; but, as the 

 river approaches its mouth, the flow becomes slackened and 

 the sediment subsides. Hence, in the lower part of the 

 course of the Thames, especially in the " reaches," or bends 

 in the river, near Woolwich, there are large mud banks ; 

 and this mud is systematically dredged up and removed, in 

 order to prevent obstruction. Those particles of mud which 



