"Yes," was the reply. "If there was no air in the glass it would have to be very many times as long as this glass is to hold the water that would rise if it had a chance. But come, let us sit down on the steps again, and I will tell you about it." When they were settled he continued: "Over two hundred and fifty years ago there lived a man named Galileo, who learned a great many wonderful things by studying the stars and doing just such things as we have been doing. It was he who made the first thermometer. But there was one question that he could not answer. He found that in a hollow glass tube, closed at one end, water would rise thirty-four feet high, but no higher. He could not tell why. A pupil of his thought he would try the same thing with the heaviest liquid known----" "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. "Yes," was the reply, "simply a glass tube about thirty-three or thirty-four inches long, closed at the top, and filled with mercury. It is then placed in a small open cup, called the cistern, into which the mercury flows until the air pressing on it there will let it fall no farther." "Does it always stay at the same height in the tube?" asked Donald. "Oh, no," his uncle answered. "Some days the air is heavier than others, and so presses harder on the mercury." "That would make it rise, wouldn't it?" asked Susie. "Yes, dear." "So, uncle," said Frank, taking up the Weather Report, "where it says 'High' here, it means that the air is heavier than where it says 'Low.' Is that it?" "That's right," replied Uncle Robert; "and when the barometer is low we know there will be a storm." "Well"--and Donald stood up and stretched himself--"I wish I could see a barometer." "You shall," said Uncle Robert "I will send for one. You may carry the letter to the post office to-morrow when you go for the mail." CHAPTER VIII. A WALK IN THE WOODS.