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 You had to lift the air out of the hollow pump before the water could come in. Pumping air is hard work. There are several simple ways of proving that air weighs something. Empty the teakettle of water and take the lid off. Now lay a sheet of rubber cloth over the top. Put a glass or rubber tube into the spout, and draw the air through the tube into your lungs. Breathe again. Suck all the air out of the kettle. See the rubber cloth sink. The air on top is pushing it into the kettle. There it goes, down inside. There is no air in the kettle to hold it up.

Now blow into a little rubber balloon. See it swell. Blow again, the rubber stretches. See how much air you can blow into it. Ah, it burst! There was more air inside the balloon than there was outside. Fill a glass with water until it overflows. Cover the top with a sheet of thick, smooth letter paper. Press it all around the edges so you are sure no air can get between the glass and the paper, to let the water out. Now take hold of the glass by the bottom and turn it upside down. The water will not spill, the air below holds it up. Turn back to Air in this book and find out what air weighs.

Just as fish and sea plants may live at the bottom of the ocean of water, so land animals and plants live at the bottom of the ocean of air. The top of this ocean of air is level, too. Do you think the air on a mountain peak would be as deep or as heavy as the air in a low valley? A glass full of water has no color. But an ocean full of water looks blue. We cannot see air itself, but we can see the color of it. When it is forty miles deep air looks blue, too.

Robert likes to climb trees, so he is just the boy to get up on a step ladder and take down those dusty curtains. Warm up there, Robert? Hot air goes up, so it is always warmer near the ceiling. You remember how warm air carries vapor up to make rain? It is the hot air that carries smoke up from chimneys. Did you ever send up a red paper balloon on the Fourth of July? You lit a tiny candle at the bottom. Soon the sides of the balloon swelled and stretched tight. Air expands when it is heated. That is, it