In countries where the winters are cold it is necessary to devote a great deal of time and labor to the heating of dwellings. Heat is usually obtained by the? burning of wood, coal, etc. Such substances are called fuel. The harder the fuel, the more difficult it is to kindle. Coal is harder to light than wood because of its density, which increases the difficulty of raising it to the temperature which is necessary for burning. If the heat of another fuel, such as kindling wood, be applied to the coal in sufficient quantity and long enough to ignite it, it will then produce a fire much more powerful and much more durable than will the lighter fuel. Lighter fuel kindles easily, but the mixture of air in its pores causes it to burn out rapidly. Hence the heat it produces is but temporary, though often very strong. The usual method of getting rid of the smoke from a fireplace is through a chimney.