The winds in passing over the land may be either cooled or warmed, and hence their influence on evaporation is continually changing; but the mean rate of evaporation from an open water body can be determined for a definite time, say a year, for various localities, and thus afford a means of comparison between one region and another. Observation of the mean annual evaporation for various stations, mostly within the United States, have been made, and the result shown by lines drawn through places where the rate is the same. A map showing this data, on which the figures indicate the depth of evaporation in inches, is here presented. The systematic study of evaporation, and especially the part played in it by plants, has scarcely more than been begun on this continent, and important results concerning its influence on atmospheric conditions are to be expected. The subject is also of great importance in reference to agriculture, the prevention of frost, etc. [Illustration: FIG. 27.--Depth of evaporation. After T. Russell.] LITERATURE The great storehouse of information pertaining to the weather and climate of the United States is the numerous publications of the United States Weather Bureau, Washington, D. C. Similar bureaus exist at the capitals of Canada and Mexico, which have issued valuable reports. Of the many elementary and popular books on meteorology, the following will be found helpful in continuing the study of the subjects outlined in this chapter: DAVIS, W. M. _Whirlwinds, Cyclones, and Tornadoes_, Lee & Shepard, Boston, 1884; _Elementary Meteorology_, Ginn & Co., Boston, 1894. FERRIL, W. _A Popular Treatise on the Winds._ Wiley & Sons, New York, 1889. GREELY, A. W. _American Weather._ Dodd, Mead & Co., New York, 1888. _Maryland Weather Service_, vol. i, Baltimore, 1899. Contains a valuable bibliography. RUSSELL, T. _Meteorology._ Macmillan & Co., New York, 1895. SHALER, N. S. _Aspects of the Earth._ Scribner's Sons, New York, 1889. WALDO, F. _Modern Meteorology_, Scribner's Sons, New York, 1893; _Elementary Meteorology_, American Book Company, New York, 1896. CHAPTER IV PLANT LIFE