The Fairy-Land of Science

But how come they to find any empty space to receive them? To answer this we must go back again to our little active invisible fairies the sunbeams. When the sun-waves come pouring down upon the earth they pass through the air almost without heating it. But not so with the ground; there they pass down only a short distance and then are thrown back again. And when these sun- waves come quivering back they force the atoms of the air near the earth apart and make it lighter; so that the air close to the surface of the heated ground becomes less heavy than the air above it, and rises just as a cork rises in water. You know that hot air rises in the chimney; for if you put a piece of lighted paper on the fire it is carried up by the draught of air, often even before it can ignite. Now just as the hot air rises from the fire, so it rises from the heated ground up into higher parts of the atmosphere. and as it rises it leaves only thin air behind it, and this cannot resist the strong cold air whose atoms are struggling and trying to get free, and they rush in and fill the space. One of the simplest examples of wind is to be found at the seaside. there in the daytime the land gets hot under the sunshine, and heats the air, making it grow light and rise. Meanwhile the sunshine on the water goes down deeper, and so does not send back so many heat-waves into the air; consequently the air on the top of the water is cooler and heavier, and it rushes in from over the sea to fill up the space on the shore left by the warm air as it rises. This is why the seaside is so pleasant in hot weather. During the daytime a light sea-breeze nearly always sets in from the sea to the land.