284 ENVIRONMENTAL CONDITIONS. 



with atmospheric evaporating power, since this will be readily under- 

 stood and since an attempt to^ avoid such practice seems somewhat 

 pedantic at the present time. When the evaporating power of the air 

 begins to enter seriously into climatogical studies this meaning of the 

 word ''evaporation" may be dropped. In short, it seems desirable to 

 avoid clouding the main issue for the present, and we shall frequently 

 employ the word "evaporation" to correspond with the word "pre- 

 cipitation" as here used. The expression "evaporating power of the 

 air," or "atmospheric evaporating power," will also be used, however. 



(2) Atmospheric Evaporating Power. 



■ This term is here used in Livingston's^ sense, meaning the tendency 

 of the air about the plant to accelerate transpiration. The expression 

 has been seriously opposed as an "inaccurate and misleading expres- 

 sion," by certain members of the staff of the United States Weather 

 Bureau. Some of the discussion that has been raised in this connection 

 is indicated in one of Livingston's papers (1915, 2) and in footnotes, 

 editorial and otherwise, incorporated therein. 



The objection to the term, as so far brought out in the Uterature, 

 seems to reside mainly in the consideration that the non-aqueous gases 

 of the air actually hinder vaporization of water (which they do to a 

 comparatively slight degree), so that a decrease in the amount of these 

 gases present must increase the evaporating power of the air. It does 

 not appear, however, that this is really an objection to the term in 

 question, especially since the meaning is understood immediately by 

 everyone, and since no better term seems available as yet. Doubtless 

 some of the misunderstanding brought out in this discussion hinges 

 on what may be meant by air. The air is a mixture of varying 

 proportions of various gases; it always contains (in nature) nitrogen, 

 oxygen, and a little argon, but it also contains carbon dioxide and 

 water-vapor, and frequently numerous other gases. We see no reason 

 for not considering these last-named gases as a part of the mixture, and 

 it is in the sense of air as the gas mixture bathing the evaporating sur- 

 face in question that the word has been employed by Livingston and 

 is here employed. Now, such a gas-mixture as the air may vary in the 

 nature and proportions of its constituents, and it may also vary in its 

 density, or pressure. As the pressure decreases the air becomes less 

 dense, and this purely physical change makes it possible for evapora- 



^ In this connection, see the following papers: Livingston (1906, 2). — Idem, Evaporation as a 

 climatic factor influencing vegetation, Hort. Soc. New York, Mem. 2: 43-54, 1910. — Idem, 

 A schematic representation of the water-relations of plants, a pedagogical suggestion. Plant 

 World 15: 214-218, 1912.— Atmometry and the porous cup atmometer. Plant World 18: 21-30, 

 51-74, 95-111, 143-149, 1915. — Idem, Atmospheric influence upon evaporation and its direct 

 measurement, Monthly Weather Rev. 43: 126-131, 1915. — Idem, A modification of the Bellani 

 porous plate atmometer, Science, n. s., 41: 872-874, 1915. — Idem, A single climatic index to 

 represent both moisture and temperature conditions as related to plants, Physiol. Res. 1 : 421-440, 

 1916. — Idem, Atmospheric units, Johns Hopkins Univ. Circ, 160-170, Mar., 1917. 



