Now, how is it that the atmosphere should sometimes be warm and sometimes cold? Where does the heat come from? and how does the air take it up? Let us return again to the illustration of the house. In winter, when the air is keen and frosty outside, it is warm and pleasant indoors, because fires are there kept burning. The burning of coal and wood produces heat, and the heat thus given out warms the air. Hence it is by the giving off or radiation of the heat from some burning substance that the air of our houses is made warmer than the air outside. Now, it is really by radiation from a heated body that the air outside gets its heat. In summer, this air is sometimes far hotter than is usual in dwelling-houses in winter. All this heat comes from the sun, which is an enormous hot mass, continually sending out heat in all directions. But, if the sun is always pouring down heat upon the earth, why is the air ever cold? Place a screen between you and a bright fire, and you will immediately feel that some of the heat from the fire place has been cut off. When the sun is shining, expose your hand to its beams for a time, and then hold a book between the hand and the sun. At first, your skin is warmed; but the moment you put it in the shade, it is cooled again. The book has cut off the heat which was passing directly from the sun to your hand. When the atmosphere is felt to be cold, something has come in the way to keep the sun’s heat from directly reaching us. Clouds cut off the direct heat of the sun. You must often have noticed the change of temperature, when, after the sun has been shining for a time, a cloud comes between it and the earth. Immediately a feeling of chilliness is experienced, which passes off as soon as the cloud has sailed on, and allowed the sun once more to come out. The air itself absorbs some of the sun’s heat, and the greater the thickness of air through which that heat has to make its way, the more heat will be absorbed. Besides this, the more the rays of heat are slanted the weaker do they become. At noon, for example, the sun stands high in the sky. Its rays are then nearest to the vertical, and have also the least thickness of air to pass through before they reach us. As it descends in the afternoon, its rays get more and more slanted, and must also make their way through a constantly increasing thickness of air. Hence the middle of the day is much warmer than morning or evening. At night, when the sun no longer shines, its heat does not directly warm the part of the earth in shadow. That part not only receives no heat from it, but even radiates its heat out into the cold sky. Hence night is much colder than day.