Ask your teacher to show you how the school-room is ventilated; and when you go home, talk to your parents about the ventilation of the house in which you live.
~8.~ Many people ventilate their houses by opening the doors and windows.

This is a very good way of ventilating a house in warm weather, but is a very poor way in cold weather, as it causes cold draughts, and makes the floor cold, so that it is difficult to keep the feet warm.

It is much better to have the air warmed by a furnace or some similar means, before it enters the rooms.

There ought also to be in each room a register to take the foul air out, so that it will not be necessary to open the windows.

This register should be placed at the floor, because when the pure air enters the room warm, it first rises to the upper part of the room, and then as it cools and at the same time becomes impure, it settles to the floor, where it should be taken out by the register.
~9.