872 



SCIENCE 



[N. S. Vol. XLI. No. 1067 



of quadratic vector functions. Analysis is 

 represented by a contribution from G. A. 

 Bliss on functions of lines and one from A. 

 Dresden upon tbe calculus of variations. 



Edwin Ekwell Wilson 



SPECIAL ARTICLES 



A MODIFICATION OF THE BELLANI POROUS PLATE 



ATMOMETER 



A RECENTLY renewed and increased interest 

 in tbe direct measurement of the evaporating 

 power of the air'- has brought the atmometer 

 (incorrectly called evaporimeter, etc.) into 

 greater prominence than has heretofore been 

 enjoyed by that instrument. Atmometers are 

 now being used more than ever before, espe- 

 cially in plant physiology, plant and animal 

 ecology, and the agricultural aspect of clima- 

 tology.- This newer development of atmometry 



1 The evaporating power of the air may be 

 thought of as the reciprocal of the resistance of- 

 fered by the air to evaporation from exposed liquid 

 water. The term is a misnomer to a degree, for 

 evaporation proceeds from a water surface in spite 

 of the conditions obtaining in the surrounding air. 

 Ceteris paribus, the greater is the air pressure, the 

 less rapid is evaporation; the less water is contained 

 in the air the more rapid is evaporation, etc. The 

 term has come into rather general use, however, and 

 may as well stand till a better is suggested; it is 

 logically no worse than the word suction, and, like 

 it, is readily understood by everybody. Condensing 

 power or water supplying power can not here be 

 used in place of resistance, because air without 

 water-vapor offers resistance to evaporation but 

 has no condensing power; it can not deposit water 

 upon a surface, no matter what its pressure may 

 be. The resistance offered by such dry air can be 

 expressed in terms of an equivalent condensing 

 power, however. 



2 Livingston, B. E., ' ' The Relation of Desert 

 Plants to Soil Moisture and to Evaporation," Car- 

 negie Inst. Wash. Pub. 50, 1906. Shelford, V. E., 

 "Animal Communities in Temperate America," 

 Geog. Soc. Chicago Bull. 5, 1913, pages 162-65. 

 Livingston, B. E., and) L. A. Hawkins, "The 

 "Water Relation between Plant and Soil, ' ' Carnegie 

 Inst. Wash. Pub. 204, 1915 (the first paper of that 

 publication). Shive, J. W., "An. Improved Non- 

 absorbing Porous Cup Atmometer," Plant World, 

 18: 7-10, 1915. Livingston, B. E., "Atmometry 



has emphasized the employment of water- 

 impregnated solids to furnish the evaporating 

 surface from which the rate of evaporation is 

 studied, and has discouraged the use, for many 

 purposes at least, of the open pan or tank of 

 water so commonly met with in meteoro- 

 logical and general climatological literature. 

 It has thus come about that considerable mis- 

 understanding has arisen as to what atmom- 

 etry is really aiming at and as to the relative 

 desirability of studying evaporation from one 

 or another kind of evaporating surface. 



Of all the different forms of water-impreg- 

 nated surfaces employed in the study of atmos- 

 pheric evaporating power, the cylindrical por- 

 ous clay cup of Babinet^ has met with the most 

 favor among biological and agricultural work- 

 ers, and the standardized porous cups now in 

 general use foUow the principle of Babinet's 

 device. This type of atmometer possesses a 

 number of pronounced advantages over the 

 free water surface, when atmospheric evapo- 

 rating power is to be studied as an environ- 

 mental condition affecting animals and plants. 

 Among these advantages may be mentioned 

 the fact that the evaporating surface of the 

 cup projects up into the air like most animal 

 and plant surfaces. Thus it does not so 

 readily become clogged, as it were, by its own 

 vapor blanket, as does a horizontal surface. 

 Furthermore, the porous cup instrument is 

 much more readily and precisely read than is 

 the open tank, and very short time intervals 

 may consequently be employed. I have fre- 

 quently constructed graphs showing the march 

 of evaporating power by minute or 5-minnte 

 rates, a procedure hardly possible with any 

 form of pan atmometer. More important than 

 any other of the advantages here in question, 

 however, is the one depending upon compara- 

 tive variability of the evaporating surface with 



and the Porous Cup Atmometer," Plant World, 

 18: 21-30, 51-74, 95-111, 143-149, 1915. Living- 

 ston, B. E., "Atmospheric Influence upon Evapo- 

 ration and Its Direct Measurement," Mo. Weather 

 Bev., 1915. McLean, F. T., "Relation of Climate 

 to Plant Growth in Maryland," Mo. Weatlier Bev., 

 1915. 



3 Babinet, J., ' ' Note sur un atmi 

 Compt. Bend. Paris, 27: 529-30, 1848. 



