"It proves that all the air in the box would weigh an ounce."" said Norah.
"Yes, that"s quite right," said Fred, "and so now you know that air, like every other substance, has weight.
"Now I want you to look carefully. I will fill this tumbler under the water, and then turn it upside down in the basin. The water stands in the tumbler, and does not run out into the basin. The reason is that the air in the room is pressing down on the water in the basin, and this prevents the water in the tumbler from flowing out.
"We know then that air presses downwards. "Teacher showed us another veryclever thing. I"ve tried, but I can"t do it yet. He filled the tumbler quite full of water, covered it witha piece of writing-paper, and then turned it upside down.
When he took his hand away
there was the paper, but the water did not run out of the glass.
"Teacher says the air in the room was pressing upwards against the paper. It must have pressedhard, for the whole weight of the water in the tumbler was resting on the paper, and yet it didn"t fall out.
"This proves then that air presses upwards.
