"We may have one some time that will be more like yours," said Uncle Robert, "for these records are made at eight in the evening as well as in the morning." "Uncle," said Frank, looking closely at the map, "here it says 'High,' and there it says 'Low.' What does that mean?" "It means," said Uncle Robert, "that here there is a low barometer, and there the barometer is high." "Barometer," said Donald. "What is a barometer, uncle? Is it like a thermometer?" "Well, not exactly," was the reply. "With the thermometer, you know, we tell the temperature of the air, and with the barometer we tell how heavy it is." "How heavy the air is!" exclaimed Susie. "How funny! Why, uncle, air doesn't weigh anything, does it?" "More than you think, little girl," said Uncle Robert, smiling. "But perhaps we can prove whether it does or not. Frank, will you get a pail of water? Donald, see if you can find a cork some place; and Susie, run in and get a tumbler." When all was ready Uncle Robert asked Frank to fill the pan with water, and Donald to put the cork into it. [Illustration: Experiment No. 1.] "There," said Donald, as the cork floated about on the pan of water. "But I want the cork on the bottom of the pan," said Uncle Robert, "not on the top of the water." "It won't stay there," declared Donald, pushing it into the water again and again with his finger. "It is too light. Corks always float." "How can we make it go to the bottom?" No one could tell. The children looked puzzled. "Let us see what this will do," and, taking the glass from Susie's hand, Uncle Robert turned it over the cork, pressed it down into the water as far as it would go, and held it there. Looking through the glass, they could see the cork lying on the bottom of the pan. "Why, Uncle Robert!" exclaimed Susie, "what--how--" "It's the glass that does it," declared Donald. "But the glass doesn't touch the cork," objected his uncle. "There's air in the glass," said Frank, who had been looking at it quietly as the others talked. "That is what presses it down." "If it's air," said Donald, "why didn't it go down before the glass was put over it? There was just as much air about it then, and more, too." "Let go of the glass, uncle," said Frank, "and see what it will do." Uncle Robert did so, and the glass instantly turned over, while a big bubble of air escaped through the water. "There," said Frank, smiling, "I told you so!" "Then air only presses on things when there is something like the glass to hold it down. Is that so, uncle?" asked Donald. "Let us see," was the reply. [Illustration: Air Pressure. Experiment No. 2.] Filling the glass with water, he placed a piece of paper over it, and quickly turned it upside down. Not a drop of water fell from the glass. The paper, now beneath the water, stayed there as though glued.