places they have been known to partly roll and partly carry rocks 

 weighing many hundreds of pounds. This may seem almost incredi- 

 ble when you think of the weight of large stones, but water by no 

 means finds the stones as heavy as you do. Take a stone weighing 

 several pounds and hold it under water and notice how much lighter 

 it seems. If you have a spring balance you may amuse yourself by 

 weighing the stone while it is in air and then while it is in water. 

 If while it is in air it weighs twice as much as an equal bulk of water 

 it will lose half its weight when you weigh it in water; if three times 

 as much it will lose one-third of its weight in water, and so on. And 

 so you may discover about what the density of the stone is. These 

 experiments will help you to imderstand how water can carry ofE peb- 

 bles which it has dislodged. 



You must remember, however, that it is only very swift streams 

 that can carry even pebbles to any distance. You will discover how 

 mud or very fine sand is carried farther than coarse sand, and coarse 

 sand farther than pebbles if you take a handful of gravel and throw it 

 into a glass of water. You will notice that the stones fall to the bot- 

 tom at once, while the coarse sand falls in a more leisurely fashion, 

 and the very small particles that make up mud may take hours to set- 

 tle down to the bottom. In the same way the river soon drops the 

 stones that it may have taken up, while it may carry the coarse sand 

 some distance, and the fine sand and mud is carried to great distances. 



Examine the part of the bed of a river that is exposed when the 

 river is very low. You will find that where the current has been very 

 strong there may be a gravel bank. At other places, where two cur- 

 rents meet, you may find a ridge of sand which the water has dropped 

 where it is partly stopped by the opposing currents. But wherever 

 the flow has been very gentle you will find the channel covered with 

 fine mud. 



Now we have already seen the good work accomplished by the mud 

 carried by the river. But the pebbles also play an important part in 

 the work of the river. As they are shoved along near the bottom and 

 sides they naturally tear up the ground and so set more earth and 

 mud in motion, and these, too, are carried along by the stream. Evi- 

 dently this means that the pebbles help the river to scour out its bed, 

 and thus make a wider and deeper channel. If you have grasped this 

 you reaUy know how most rivers come to exist. Where you now see 

 a mighty river with a deep, wide bed there was once but a small 

 stream, and the stones carried along enabled it to scoop out a deeper 

 and wider channel and so the process has gone on until you have the 

 present river, with its great, wide, deep bed. Have you ever seen a 



»No. 16. 



