14 AGRICULTURE. 



power is supplied by the volume and velocity of 

 the stream, but the work of abrasion is per- 

 formed, for the most part, by the sand, pebbles, 

 and rock fragments as they are rolled along. 

 They cut down into the river-bed, wearing it 

 deeper; they polish each other into rounded or 

 flattened shapes, or grind each other into pow- 

 der in their mad rush to the sea. 



The transporting power of running water 

 varies as the sixth power of its velocity, so that 

 if its velocity be doubled it can carry sixty-four 

 times as much solid matter as before. Thus it 

 is that a slight increase in the velocity will 

 greatly increase the load of a stream if the ma- 

 terials are obtainable, while the slightest de- 

 crease in the velocity will cause a part of the 

 load to be deposited. These river deposits are 

 commonly in sheets or bars, but when the river 

 suddenly enters a plain at the foot of a steep 

 slope an alluvial fan is formed by the deposition 

 of the sediment. 



According to the calculations of the United 

 States government made many years ago, the 

 Mississippi River transports to the gulf every 

 year enough solid substance to make a column 

 one mile square and 268 feet high 200,000,000 

 tons. 



The student can find no better example of 

 the carrying power of water than that of the 

 roadside rills and gullies after a heavy rain. 



