英文科學(xué)讀本 第三冊(cè)·Lesson 28 More about the Air

Lesson 28 More about the Air

You remember our talk about the water bottle and the basin of water, Norah? asked Fred. "What did we learn from it?"

We learned that air is an actual substance, and that it takes up space although we cannot see it. The bottle was full of air, and that was why the water would not go in. When you slanted the bottle the air bubbled out, and the water went in.

That's very good, said Fred, "and you must remember that there is air everywhere—in every nook and corner. What we call 'empty' is really 'full of air.'"

Teacher showed us a machine called an air-pump today. It will remove the air from a vessel and make it really 'empty.' He showed us too an 'empty' square box, a foot across each way, and of course it was full of air. He said that if he could pump all that air out, the box would weigh just an ounce less than it did at first. Can you tell me what that proves?

It proves that all the air in the box would weigh an ounce. said Norah.

Yes, that's quite right, said Fred, "and so now you know that air, like every other substance, has weight."

Now I want you to look carefully. I will fill this tumbler under the water, and then turn it upside down in the basin. The water stands in the tumbler, and does not run out into the basin. The reason is that the air in the room is pressing down on the water in the basin, and this prevents the water in the tumbler from flowing out.

We know then that air presses downwards.

Teacher showed us another very clever thing. I've tried, but I can't do it yet. He filled the tumbler quite full of water, covered it with a piece of writing-paper, and then turned it upside down. When he took his hand away there was the paper, but the water did not run out of the glass.

Teacher says the air in the room was pressing upwards against the paper. It must have pressed hard, for the whole weight of the water in the tumbler was resting on the paper, and yet it didn't fall out.

This proves then that air presses upwards.

Look now at my leather sucker. I make it hold to the stone, because I press out every particle of air between them. It holds because the air outside it is pressing it to the stone. If I try it on this slate, it does not matter whether I hold the slate level, place it against the wall, or sideways, the sucker will hold. The air in each case presses it against the slate.

Now we know then that air presses with equal force upwards, downwards, sideways, and in all directions.

SUMMARY

A square box, a foot across each way, holds one ounce of air. Air presses with equal force downwards, upwards, sideways, and in all directions.