"Suppose we have a talk about the air this morning," said Mr. Wilson. "The air, you know, is a material substance. It is a gas. It surrounds our earth, and covers the tops of the highest mountains. We live and move about at the bottom of an ocean of air, just as fishes and other animals do at the bottom of the sea.
"This ocean of air, being a gas, has no surface. You remember, no doubt, that one of the points that distinguish gases from liquids is that, while liquids always keep a level surface, gases have no surface at all. This air extends far beyond the mountain tops-probably fifty milesupward-perhaps even beyond that.
"People who go up in balloons find that the air becomes thinner as they ascend, until it is very difficult, and at last quite impossible, for them to breathe, and yet the greatest height ever reached in a balloon is about seven miles.
"Now I want you to think for a moment of one of the lightest substances we have met with in our lessons-that thin, fleecy eider-down. Picture to yourselves a mass of this light, fleecy down, piled into a great heap, reaching upwards for miles. Light as the down is, it has someweight, and the upper portion must of necessity press upon that which is beneath it. What do you think would be the result of this?""The lowest part of the heap would be pressed downwards by the weight of that above it, sir. It would be packed close and dense.""You are quite right," said Mr. Wilson, "and it is just so with the air. This, being a material substance, has weight. That is to say, every particle of air is attracted or drawn down to the earth by the force of gravity. The layers above press upon those below, so that the air at the foot of a mountain is always denser and heavier than that at its summit.""It is the same with water, sir," said Fred. "You proved that the pressure of water is always greater at the bottom than near the top.""I am glad you can see the connection, Fred, between the water and the air in this respect, for the two are very similar. Air, like water, is a material substance and has weight. It is the weight in each case which presses downwards.
"Let us have another look at this instrument. I showed it to you once before. It is called an airpump. I do not intend to enter into any explanation of the airpumpat present. That will come in a lesson by itself later on. All I want you to know about it now is that, with the help ofit we are able to remove the air from a vessel, and leave it empty. If we were to take a square box measuring a foot each way, we should find, by first weighing it, then exhausting the air fromit, and then weighing it again, it would lose rather more than one ounce. That is to say, a cubic foot of air weighs about one ounce.
"But if we took our square box and the airpump up the side of a mountain, and repeated the experiment, we should find that the cubic foot of air would not weigh an ounce there, and it would weigh still less as we approached nearer the top, because the air becomes lighter and thinner as we ascend."Lesson 32
