"You are right, Frank. The great ocean in which we live is the air, or, as it is called, the atmosphere. The atmosphere is just as much a part of our world as the rock and the water. The rock we may call solid, the water fluid, and the air gaseous. Solid, fluid, gas."
"How do we know that the atmosphere is so deep?" asked Frank.
"We do not know exactly, but there are ways of proving that it is very, very deep. When people began to study the atmosphere they thought it extended about fifty miles from the surface of the earth. Now they are sure that it is much deeper. We know that air has weight, like soil and water. It presses on us and everything else—"
"Fifteen pounds to the square inch," said Donald.
"We weigh the air with the—-"
"Barometer," said Susie.
"It is heavier at the ocean level than it is on the tops of mountains.
We are sure that the higher we go up—-"
"The less the air weighs," said Frank.
"At the height of fifty miles it is thought to have little or no weight, and so people believed that was as far as it extended. But in time they discovered another way of measuring the atmosphere. You have seen falling stars, haven't you?" asked Uncle Robert.
[Illustration: Meteors.]