"Uncle," said Frank, "is it truly the air that holds the paper on and keeps the water in the glass? If it presses that way everywhere, why don't we feel it?" "It is because it presses equally in every direction," replied Uncle Robert. "Put your hand in this pail of water. Do you feel it pressing on your hand?" "No," said Frank. "Place it lower in the water. Does it feel any heavier now?" "Not at all," answered Frank. "But you know that the water is heavy. Lift the pail, Donald." "It is heavy," said Donald, setting it down. "I don't see why Frank didn't feel a little of the weight of it when his hand was under all the water." "It is this way," explained Uncle Robert. "The water pressed on his hand from below as much as from above, and the same on both sides. When you lifted it you felt its weight pressing downward only. Now it is just so with the air. It presses with such equal pressure that we do not realize its weight. It is only when it presses harder from one direction than from another that we feel it." "That's when the wind blows, isn't it, uncle?" asked Donald. "Yes, my boy," was the reply. "You can see how it is out among the trees now." "But, uncle," said Donald, "how can the air be weighed if it presses the same in all directions? It was only when I lifted the whole pail of water that I felt how heavy it was. The air can't be weighed if it presses up just as much as it does down." "But if in some way it could be shut off so that it would only press in one direction?" "It might be," answered Donald, "but I don't see how." Uncle Robert told Susie to put the glass in the water so that it would all be below the surface, and, without taking it from the water, to turn it upside down. She did so, and then began to lift it slowly out of the water. "See," cried Susie, "the water comes with it. The glass is full. Could I lift it clear out that way?" "Try it," said Uncle Robert, smiling. But no; when the edge of the glass came out of the water in the pail, down went the water with a splash. "I see how it is," said Frank, who had watched it closely. "There wasn't any air in the glass to keep the water out, as there was when we turned it over the cork, so the water stayed in it." "But what made it come up out of the pail?" asked Donald. "There wasn't any air under it to press it up." "Would the air pressing on the water around the glass make it do so, uncle?" asked Frank, placing the glass in the water and raising it as Susie had done. "It seems as if it might be that." "That is what it is," replied his uncle. "The air pressing on the water in the pail forces it into the glass, where there is nothing to keep it from rising." "If the glass was longer would the water stay in it just the same?" asked Donald.