Look at the trees when the wind blows, and see the clouds fly. They show that the air is moving fast, and we say, "The wind is blowing." But in the house the air moves more quietly, though even here the warm air and the cold are always changing places.

Put a soft feather above a lighted lamp or gas jet and see it fly toward the ceiling. Hot air is lighter than cold air, and it gets so very hot around the lamp that it goes up with a rush, taking the feather with it. Soon, however, the air grows cooler as it leaves the lamp, and the feather comes down again.

Breathe on your hand, and you will see that your breath is warmer than the air in the room. You will know from this that your breath rises, too.

Since all this warm air goes to the top of the room as fast as it can, the air up there must be warmer than it is on the floor. To find out about this, you might set a thermometer on the floor for fifteen minutes and see how high the mercury goes. Now put the same thermometer on top of the bookshelves for another fifteen minutes and look at the mercury again. In winter you will find quite a difference between the two places, for at that time hot air is sent into the room and it rises to the ceiling first.

In summer, when the windows are open, there is not so much difference between floor and ceiling air as during the winter.

I stopped my writing just now to make an experiment in the warm room where I sit. I opened the door into the cold hall a narrow crack, then lighted a match and held it close to the crack in the door, near the floor.

Instantly the flame was bent far over into the room. This showed that down there the air was hurrying from the cold hall into my warm room.

Next I held the lighted match against the open crack, near to the top of the door. Thereupon the flame went through the crack and tried to get out into the hall. From this I knew that up there the warm air was going out into the cold hall as fast as it could squeeze through the crack of the door. The whole experiment shows how the air hurries back and forth through any crack it finds.

Since the air moves so easily in one direction and another, it is never hard to get fresh air into a house from out of doors, and it is no harder to send impure air out of doors from any room or any house. All we have to do is to keep the matter in mind and arrange for it.

We must also remember that in any room impure air mixes with pure air fast enough to spoil the whole of it in a very short time. We cannot tell the condition of the air by the looks of the room because we cannot see the air itself, but a little experiment with ink and water will help us out.

Put ten drops of ink into a glass of water and it will mix so quickly that you will have rather poor stuff for drinking. Put in fifty drops and the water will be so black that nobody would think of drinking it. This is about the way that impure air from the lungs of fifty children mixes with pure air in a schoolroom and spoils it for breathing. As this mixing goes on all the time, we need to know just how much fresh air we ought to have to keep the whole of it pure enough to breathe.

You might measure something that is one foot long, one foot high, and one foot wide. A box as big as that will hold one cubic foot of air.

Now men who know about it say that, if possible, a child should have two thousand cubic feet of fresh air every hour. This means that, if you could use the box for a spoon, you would have to put two thousand spoonfuls of air every hour from out of doors into the room where you are. More than that, you would have to make room for it by dipping just as much impure air out of the room.

Each child needs as much as you do, and grown people need more. This shows how much air must come and go every hour to supply the fresh air needed in a room full of school children.

In almost every city there are old schoolhouses where the rooms do not get fresh air enough; but in New York City the new schoolhouses are made in such a way that each child has eighteen hundred cubic feet of fresh air every hour.

Albert S. Lyons

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