第95章 第二册(12)

"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 passthrough its pores, but holds back all the mud." "That"s quite right, Norah." said Fred. "Now I"m going to makea 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 witha 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.