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Rh or slowly, as in the case of non-metals. The former are good conductors; the latter are sometimes regarded as poor conductors, or insulators. Thus, steam pipes are wrapped with asbestos coverings. The metal pipe itself warms rapidly and, radiating heat rapidly, causes a loss of heat in the steam. The asbestos covering, being a non-conductor, or insulator, prevents the loss by radiation. In solid bodies, the diffusion of heat is accomplished by conduction. The motion imparted to molecules sets the molecules nearest to them in motion; finally the heat is diffused throughout the mass.

When liquids and gases are heated a movement of mixing occurs. This process is best observed when a handful of sawdust is placed in a beaker of water and the water is heated by a Bunsen burner. The rapid warming of the water carries the particles of sawdust upward, outward and downward through the water; they indicate the progressive movement of the water in different parts of the beaker. This mixing process is called convection.

Convection of the Air.—The convectional movements of the air are among the most important factors in weather science. Aside from the lateral movements of the air—the winds—convectional movements that are more or less vertical are going on all the time; that is, air is going up or coming down. Warm air is ascending, cool air is descending.

The sun does not warm all parts of the earth evenly. In tropical latitudes the heat is far more intense than in extra-tropical latitudes. Because of the curvature of the earth’s surface, polar regions receive the sun’s rays very obliquely. The unequal heating results in a convectional movement of the air on a scale that affects the whole atmosphere. It produces an upward and poleward flow of air in tropical regions which is balanced by a downward and tropic-ward movement of the air in extra-tropical regions.

The principle of convection is one of the most important in meteorology, and is practically the foundation of that