英文科學讀本 第四冊·Lesson 33 Pressure of the Atmosphere

Lesson 33 Pressure of the Atmosphere

Come, boys, said Mr. Wilson, "I’ve got a plaything. I want to be a boy again. You, of course, know what it is. It is a leather sucker. I have had it soaking in water for some time, so that it is quite ready for us. Suppose you come here, Fred, and try it on this slate.

Ah! I thought you would know how to use this toy, he continued, as Fred pulled the string and lifted up the slate. "The leather seems to have stuck to the slate, but we know that is not the case, because leather is not an adhesive substance. Tell us what you did to make the leather hold to the slate like this, Fred."

When I put the sucker on the slate I pressed it down close, sir, that is all, said Fred.

Quite right, and in pressing it you squeezed out all the air from between the leather and the slate. There is nothing between them, not even a little air. There is air outside, but there is none under the sucker. This is the whole secret. The sucker is held fast to the slate by this outside air pressing down upon it. We will now remove the sucker, place the slate against the wall, and in that position try it once more. Yes, the sucker again holds fast, and of course it is the air that is pressing it to the slate just as before. If I hold the slate overhead, or slant it in any direction, the result will still be the same. The sucker will hold fast, and in each case the cause will be the same—the pressure of the air on the outside of it.

This being so, what have we discovered?

The air presses equally in every direction, sir, for it does not matter how the slate is placed, the sucker always holds fast.

Now watch what happens when I bore a hole in the sucker, said Mr. Wilson. "The sucker is at once released. Some air rushed in through the little hole, as soon as it was made, and this air pressed upwards with as much force as the outer air was pressing downwards.

Let me show you another experiment now, he added. "I have here a tumbler filled to the brim with water. I will place this piece of writing paper over the glass, and invert it. When I remove my hand the paper remains close to the glass, so that the water cannot run out. What is the cause of this?

The air presses upwards on the paper and holds it close to the glass. This pressure of the air, too, must be considerable, for the water in the tumbler is heavy, and would naturally fall if unsupported. Now I will place the tumbler, inverted as it is, in this bowl of water with the lower end just below the surface. When I remove the paper, what do you see? The water still stands in the tumbler, although there is now no paper to keep it in. There is only the water in the basin.

Shall I tell you why the water does not rush out of the glass? It is because the air in the room is pressing downwards on the surface of the water in the basin, and this downward pressure is transmitted in all directions by the water. The water in the basin presses upwards into the water in the tumbler, and prevents it from running out.