The action of carbon dioxide in affecting climate rests on a simple physical basis; the gas is readily penetrated by the direct rays of the sun, but the heat waves reflected by the earth are stopped, so that the carbon dioxide forms a blanket which keeps in the warmth received from the sun. The other constituents of the atmosphere have similar properties, but to a less extent, and it is the carbon dioxide which has had a controlling influence on climate in past times.
Water vapour is dissolved in the atmosphere, and the warmer the air the more water vapour can it hold. When a warm mass of air is brought into contact with colder air, the moisture held in the former can no longer be held dissolved, and hence it condenses and falls as rain. The familiar form of this is when the barometer falls: the cold upper layers of the atmosphere are brought lower, and round the edge of the depression the warmer air near the earth is cooled and rain falls. Water vapour is absorbed into the atmosphere in two ways: (a) from the surface of the ocean and other bodies of water; (b) from transpiration of moisture through the pores of plants, and also from the corresponding breathing and perspiration in animals. Most of the water absorbed from the ocean is precipitated back into the ocean as rain; the water vapour from which most of the land rains are derived comes from the transpiration of plants. A field covered with lucerne or meadow grass gives off some 400 lb. of water a month from every square yard; wheat and other cereals about half as much; and potatoes, vines, and such like about 50 lb.; oak and fir forest about the same. It is evident, therefore, that the more cultivation there is, and the more forest there exists in a land, the more rain will there be. In South
