"That was mercury, wasn't it, uncle?" interrupted Donald.
"Yes; he used mercury, and found that it rose in the tube just thirty inches. He knew that the mercury was thirteen and six-tenths times as heavy as the water, so he felt sure that it was the pressure of the air that made them both rise in the tube, for thirty-four feet is just thirteen and six-tenths times thirty inches. But they wanted to see if it was really the air, so they took the tube up on a high mountain."
"What difference would that make?" asked Susie.
"Look at the woodpile out there," said her uncle. "Where do you think the weight of the wood would be the greater? On the ground or halfway to the top?"
"On the ground, of course," answered Susie.
"Well, they found it was the same with the air. As they went up the mountain the mercury in the tube fell."
"That showed that the weight on it was less, didn't it, uncle?" said
Frank. "I think that was a very wonderful discovery, don't you?"
"It was, indeed," replied Uncle Robert, "and that is how the first barometer was made."
[Illustration: Barometer.]
"Is that what a barometer is?" asked Donald.