英文科学读本 第二册·Lesson 05 The Poor Man's Filter

Lesson 05 The Poor Man's Filter

I say, Fred, said Willie, "shall we tell Norah about the biggest filter in the world?"

Oh yes, do, Fred. said Norah.

Perhaps we can make Norah tell herself all about it, Will, said Fred. "You remember what a heavy rain we had last night, Norah?"

Oh yes, said Norah, "the water stood in great puddles in the road."

Where are the puddles now?

They are all gone; the water has sunk down into the earth. said Norah.

Why does the water sink into the earth?

Because the earth is porous and absorbs it, said Norah.

Quite right, said Fred. "But is all earth porous?"

No. Clay is not porous. said his sister.

You know we found a spring the other day when father took us for a walk?

Oh yes, she said, "and we had a good drink. What clear, bright, sparkling water it was, Fred."

Ah, said he, "that clear water came from some dirty puddle of rain water, which sank into the ground drop by drop. It kept sinking through gravel, sand, and rock, till it came to something like clay. It could not sink through the clay, for clay is not porous. Then it had to burst its way somewhere, and it came bubbling out of the ground as a spring."

Now I know what you mean, said Norah. "The earth itself is a filter. It lets the clear water pass through its pores, but holds back all the mud."

That's quite right, Norah. said Fred.

Now I'm going to make a filter for ourselves. It is so easy to make, and costs so little that teacher calls it the Poor Man's Filter. Father has given me this nice clean flowerpot and all the things I want. I will first plug up the hole in the bottom with a piece of sponge. Then I will cover this with a layer of small charcoal an inch or two deep. Above the charcoal I will put a layer of fine sand, about the same thickness. All these things—sponge, charcoal, and sand— are porous. We have a filter now, ready for use. But as I have some charcoal and sand left, I will put two more layers in.

Now if we pour any dirty muddy water into the top of the flowerpot, we shall see it trickle through the bottom clear and bright.

SUMMARY

The earth is a great filter. It allows the clear spring water to trickle through its pores, but keeps back all the mud.