英文科學(xué)讀本 第三冊(cè)·Lesson 18 Ice, Hail, and Snow

Lesson 18 Ice, Hail, and Snow

I think we can find something more to say about ice, said Fred. "I seem to have learned ever so much from our last lesson."

Just you try and think about the frozen ponds in winter time, Norah. Where do we find the ice?

It covers the whole surface of the pond.

But suppose we took a hammer and broke the ice on the surface. Should we find the whole of the water in the pond solid ice?

No, the water is not frozen underneath.

Can you tell me what would become of the pieces of ice as we broke them up?

They would float on the water.

You are quite right. Teacher put a piece of ice in the water and we saw it float on the top. But what does that prove?

It proves that the ice is lighter than the water, said Norah. "If it were heavier it would sink to the bottom."

Yes, said Fred, "ice is lighter than water, and it always forms on the surface. Even in a hard frost there is only a sheet of ice covering the surface of the water, and when it breaks up we can see the water below."

Suppose for a moment that the ice were heavier than the water. It would sink to the bottom as it formed, and the whole pond would in time be solid ice. What would become then of the fishes and other creatures that live in the water?

Why, I suppose they would be frozen too, and die. said Norah.

Quite right, said Fred. "As it is, the ice protects the water from the cold. Without it the fishes and water plants would all die."

I was very much surprised to learn about the hail and snow, said Willie.

All right, Will, said Fred. "Then suppose you have a turn, and tell us all you can remember about them."

Well, Norah, I suppose you know what hailstones are like? said Willie.

They are like little round balls of ice.

But what do you think they really are? he asked again. "They are nothing more than frozen raindrops. You know the rain falls in little round drops."

Teacher says that sometimes the raindrops, as they fall, have to pass through very cold air, and they are frozen into round balls of ice before they can reach the earth. The little round balls of solid water we call hail.

Snow, too, is another form of solid water. You know that evaporation is constantly going on everywhere. The vapor as it forms rises and makes clouds. The clouds we see in the sky are only masses of vapor.

Teacher says that sometimes the whole cloud of vapor becomes suddenly cold, and the tiny particles of vapor are frozen. This frozen vapor is too heavy to float in the air. It falls to the earth as snow.

SUMMARY

Ice is lighter than water. It floats on the top of the water. If it were not for this, fishes and plants would all die. Hail consists of frozen rain-drops. Snow is frozen vapor.