英文科学读本 第三册·Lesson 36 Balloons

Lesson 36 Balloons

I little thought, said Willie, "where teacher was going to lead us when he asked us to tell him all we could of the uses of coal-gas. Did you, Fred?"

No, that I certainly didn't, said Fred. "It was easy to think of the use we make of it for lighting and heating purposes. But when teacher said he was going to talk to us about balloons, I began to wonder what balloons had to do with coal-gas."

Why, of course, said Norah, "they can have nothing to do with it."

Don't you be too sure, little girl, said Fred. "You had better wait a bit."

Even then, said Will, "I couldn't make out how we were going to get any nearer to it with that cork and the basin of water."

Oh, do tell me all about it, said Norah. "I don't like puzzles."

Well then, said Fred, "teacher held a cork at the bottom of the bowl of water, and then let go."

Yes, said Norah, "and of course the cork rose to the top of the water."

Ah, but why did it rise? asked Fred. "Would a stone rise?"

The cork rose, said Norah, "because it is lighter than the water; the water pushed it up. The stone is heavier than the water and sinks to the bottom."

That was all teacher wanted us to see, said Fred, "and then he went back to the balloon. You remember seeing the men send the balloon up in the park last summer. It rose up through the air as easily as the cork rose through the water. What does that prove?"

I suppose, said Norah, "it proves that the balloon is lighter than the air. But I can't see how that can be, for there was a car hanging from it, and there were two men in the car."

Well, it certainly was lighter than the air, said Fred, "or it would never have risen and floated away as we saw it. Let us find out what made it light."

That great silk bag which we call the balloon was filled with coal-gas, and coal-gas is only about half as heavy as air. When the great bag is full of this gas, therefore, it is not only able to rise in the air itself, but it is able also to carry up with it car and men too.

SUMMARY

Balloons float in the air, because they are filled with coal-gas, which is only about half as heavy as the air. The dense, heavy air forces the balloon up, just as water forces a cork to its surface.