"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?"