Fix up the apparatus shown in Fig. 54. The small beaker A is to represent the narrow mountain stream, the larger one B stands for the wide river, and the glass jar C for the mouth of the river or the sea. Run water through them; notice that it runs quickly through A, slowly through B, and still more slowly through C: we want it to do this, because the stream flows quickly and the river slowly.
Now put some soil into A. At once the soil is stirred up, the water becomes muddy, and the muddy liquid flows into B. But very soon a change sets in, the liquid in A becomes clear, and only the grit and stones are left in the bottom: all the mud—the clay and the silt—is washed into B. There it stops for a long time, and some of it will never wash out. The liquid flowing into C is clearer than that flowing into B. If you keep on putting fresh portions of soil into A you can keep B always muddy, although A is usually clear. At the end of the experiment look at the sediment in each beaker: in A it is clear and gritty, in B it is muddy. If you can get hold of some sea water put some of the liquid from C into it: very soon this liquid clears and a deposit falls to the bottom, the sea water thus acting like the lime water on p. 20.
Fig. 54. Model of a stream. In A, where the stream flows quickly, the water is clear and the sediment free from mud. In B, where it flows slowly, the water is turbid and the sediment muddy
The experiment shows us that the fine material washed away by a quickly flowing stream is partly deposited when the river becomes wider and the current slower, and a good deal more is deposited by the action of the salt water when the river flows into the sea. The rock that crumbles away inland is spread out on the bed of the river or at its mouth.
