Hot or warm air is lighter than cold air. You have learned how heat expands bodies. It is this expansion of air, or the separation of its particles further from each other which makes it less dense or heavy than cold air, where the particles lie more closely together. As a consequence of this difference of density, the light warm air rises, and the heavy cold air sinks. You can easily satisfy yourselves of this by experiment. Take a poker, and heat the end of it in the fire until it is red-hot. Withdraw it, and gently bring some small bits of very light paper, or some other light substance, a few inches above the heated surface. The bits of paper will be at once carried up into the air. This happens because the air, heated by the poker, immediately rises, and its place is taken by colder air, which, on getting warmed, likewise ascends. The upward currents of air grow feebler as the iron cools, until, when it is of the same temperature as the air around, they cease. This is the principle on which our fire-places are constructed. The fire is not kindled on the hearth, for, in that case, it would not get a large enough draft of air underneath, and would be apt to go out. It is placed some way above the floor, and a chimney is put over it. As soon as the fire is lighted, the air next it gets warmed, and begins to mount, and the air in the room is drawn in from below to take the place of that which rises. All the air which lies above the burning coal gets warmer and lighter; it therefore flows up the chimney, carrying with it the smoke and gases. You will understand that though a bright blazing fire is a pleasant sight in winter, we do not get all the heat which it gives out. On the contrary, a great deal of the heat goes up the chimney; and, except in so far as it warms the walls, passes away and warms the outer air. What happens in a small way in our houses takes place on a far grander scale in nature. As already pointed out, the sun is the great source of heat which warms and lightens our globe. While the heat of the sun is passing through the air, it does very little in the way of warming it. The heat goes through the air, and warms the surface of the earth. You know that in summer the direct rays of the sun are hot enough to burn your face, and yet, if you put even a thin sheet of paper over your head, enough to cut off these rays, the sensation of burning heat at once goes off, although the same air is playing about you all the time. Both land and water are heated by the sun’s rays, and the same change in the air then takes place which we find also at our firesides. The layer of air next the warmed earth becomes itself warmed. As it thereby grows lighter it ascends, and its place is taken by colder air, which flows in from the neighborhood to take its place. This flowing in of air is wind.