Drainage also raises the temperature of the soil by the admission of warm air. This proposition is closely connected with that just discussed. When the air is warmer than the soil, as it always is in the Spring-time, the water from the melting snow, or from rain, upon drained land, passes downward, and runs off by its gravitation. As "Nature abhors a vacuum," the little spaces in the soil, from which the water passes, must be filled with air, and this air can only be supplied from the surface, and, being warmer than the ground, tends to raise its temperature. No such effect can be produced in land not drained, because no water runs out of it, and there are, consequently, no such spaces opened for the warm air to enter. Drainage equalizes the temperature of the soil in Summer by increasing the deposit of dew. Of this we shall speak further, in a future chapter. _Drainage raises the temperature in Spring by diminishing evaporation._ Evaporation may be defined to be the conversion of liquid and solid bodies into elastic fluids, by the influence of caloric. By heating water over a fire, bubbles rise from the bottom of the vessel, adhere awhile to the sides of it, and then ascend to the surface, and burst and go off in visible vapor, or, in other words, by evaporation. Water is evaporated by the heat of the sun merely, and even without this heat, in the open air. It is evaporated at very low temperatures, when fully exposed to the air. Even ice evaporates in the open air. We often observe in Winter, that a thin covering of ice or snow disappears from our roads, although there has been no thawing weather. In another chapter, we have considered the subject of "Evaporation and Filtration," and endeavored to give some general idea of the proportion of the rain which escapes by evaporation. We have seen, that evaporation proceeds much more rapidly from a surface of water, as a pond or river, than from a land surface, unless it be fully saturated, and that evaporation from the water exceeds the whole amount of rain, about as much as evaporation from the land falls short of the amount of rain. Thus, by this simple agency of evaporation, the vast quantities of water that are constantly flowing, in all the rivers of the earth, into the sea, are brought back again to the land, and so the great system of circulation is maintained throughout the ages. As evaporation is greatest from a water-surface, so it is greater, other things being equal, according to the wetness of the surface of any given field. If the field be covered with water, it becomes a water-surface for the time, and the evaporation is like that from a pond. If, as is often the case, the water stands on it in spots, over half its surface, and the rest is saturated, the evaporation is scarcely less, and has been said to be even more; while, if the surface be comparatively dry, the evaporation is very little.