Because water may be made to boil at its surface without melting ice a short distance below the surface. How do we know that air is a poor conductor of heat? Because the air at the ceiling of a room may be made very warm without melting ice near the floor. Why does a linen shirt feel cool? Because linen is a good conductor, and there is but little air among its fibres. Why is a cotton shirt warmer than one made of linen? Because the cotton contains more air among its fibres, and air is a poor conductor of heat. Why is a woollen shirt warmer than one made of cotton? Because wool contains more air among its fibres, and is, therefore, a poorer conductor of heat. Why is fur warmer than wool? Because there is more air among the hairs of the fur, and it is, therefore, a poorer conductor of heat. What fur is the warmest? Fur with very fine hairs, like that of the rabbit, because it contains the most air. Why is fur warmer when the hair is next to our bodies? Because the fur and the air in it both help to keep the heat of our bodies from passing off rapidly. Is the earth a good conductor of heat? No; the earth is a poor conductor of heat. How do we know that the earth is a poor conductor of heat? Because the heat of summer warms the earth only a few inches below the surface. How is it with the earth in winter? In this latitude, the frost of winter reaches only a few inches below the surface of the earth. In all parts of the earth, except in the Frigid Zones, the heat of the summer takes away the frost and ice of the winter, and the earth is warmed so that plants may grow. Why is spring-water generally cool, even in summer? Because the springs are mostly so far below the surface of the earth, that they are not warmed by the sun’s rays. Why is snow a warm covering for the earth? Because the air among the particles of snow prevents the heat of the earth from passing off rapidly. Why does wrapping straw around a pump in winter, prevent the water in it from freezing? Because the stalks of straw are hollow, and the air in the stalks makes them poor conductors of heat. Will straw, wrapped around shrubbery, prevent it from freezing? It will; because it keeps the heat of the shrubbery from passing off into the air. Why are rooms warmer from having double windows? Because the air which is confined between the double windows, being a poor conductor, keeps the warmth of the room from escaping. Why is the space between the double walls of ice-coolers filled with charcoal? Because charcoal is a poor conductor, and keeps the heat of the air from the ice. Why do ice-houses have double walls, with saw-dust or straw between them? Because the saw-dust and the straw are both poor conductors, and keep the heat of the air from the ice. Why does ice melt more slowly when wrapped in flannel? Because the flannel is a poor conductor, and keeps the heat of the air from the ice.