Earth Study 819 



stream is doing the least work. The poets, as well as common people, 

 speak of the playing of the brook when its limpid waters catch the sun- 

 beams on their dimpling surface ; but when the waters are roily the brook 

 is working very hard. This usually occurs after a rain, which adds much 

 more water to the volume of the brook; the action of gravity upon this 

 larger and heavier body forces it to flow more swiftly and every drop in the 

 stream that touches the bank or bottom, snatches up a tiny load of earth 

 and carries it along. And every drop thus laden, when it strikes against a 

 corner of the bank, tears more soil loose through the impact, and other 

 drops snatch it up and carry it on down the stream. And after a time 

 there are so many drops carrying loads and bumping along, knocking loose 

 more earth, that the whole brook, which is made up of drops, looks muddy. 

 In its work as a digger, every drop of water that touches the soil at the 

 bottom or on the banks of the brook uses its own little load of earth or 

 gravel as a crowbar or pickaxe to pry up other bits of dirt and gravel; and 

 all of the drops hastening on, working hard together, dig the channel of the 

 brook wider and deeper. In some steep places, so many of the drops are 

 working together that they are able to pick up pebbles or stones, with 

 which they batter and tear down larger pieces of the bank and scrape out 

 greater holes in the bottom of the stream. On and on the brook flows, a 

 gang of workmen each of which is using its own load as a tool, all in close 

 procession and working double quick. But as soon as the brook reaches a 

 plain or level, its activity ceases ; the drops act tired and seem to have no 

 ambition to pick up more soil, and each lets fall its own load as soon as 

 possible, dropping the larger pieces of gravel and rock first, carrying the 

 finer soil farther, but finally letting that down also. If we examine the 

 sediment of a flooded brook, we find that the gravel is always dropped first, 

 and that the fine mud is carried farthest before it is deposited. 



The roar of a flooded stream is very different from the murmur of its 

 waters when they are low. It is not to be wondered at, when we once think 

 of all that is going on in the brook during periods of flood. There are some 

 s'.mple experiments to show what the force of water can do when turned 

 against the soil. Pour water from a pitcher into a bed of soft soil, and note 

 how quickly a hole will be made ; if the pitcher is held near the soil, less of a 

 hole will be formed than if the pitcher is held high up, which shows that the 

 farther the water falls, the greater is its force. This explains why the banks 

 of streams are undermined when a strong current is driven against them. 

 The swift current, of course, breaks away more earth at bends and curves 

 than when it is flowing in a straight line; for ordinarily, when flowing 

 straight, the current is swiftest in the bed of the stream, and is therefore 

 only digging at the bottom; but when it flows around curves, it is directed 

 against the banks, and therefore has much more surface to work upon. 

 Thus it is that bends are cut deeper and deeper. If the bare arm is thrust 

 into a flooded brook, we find that many pieces of gravel strike against it; 

 and if we reach the bottom, we can feel the pebbles being moved along over 

 the brook bed. 



LESSON CCIX 



THE BROOK 



Leading thought The water from the little brook near our schoolhouse is 

 flowing toward the ocean, and is meanwhile digging out and carrying along 

 with it the soil through which it flows. 



