"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?"