365] B. E. Livingston 16'7 



and one black, when operated together, make up the radio- 

 atmometer, for use in studies of radiation as an atmometric 

 condition. 



Whatever type of evaporating surface is employed, this sur- 

 face must be clearly defined, so that the data obtained will not, 

 by any chance, be regarded as comparable with other data 

 derived from another type of surface. This means that the 

 essentials of the instrument must be described, but this can 

 be accomplished by merely naming the instrument and refer- 

 ring to some previous description. Thus, it may be stated 

 that a given set of data were obtained by means of the U. S. 

 Weather Bureau pan, the Briggs and Shantz shallow pan, 

 the Livingston standard sphere, etc. If a new type of in- 

 strument has to be used it requires a complete description. 



In stating the amount of water lost from the given instru- 

 ments during a unit of time, it is of course unimportant what 

 water unit is employed, so long as it is definite enough for the 

 work in hand. Since the whole aim of atmometry is to meas- 

 ure a power to do work, and since the amount of liquid water 

 vaporized per unit of time is considered as a measure of this 

 power, weight units rather than volume units should be used. 

 Nevertheless, if the temperature does not vary too much, from 

 reading to reading, and generally if there is no need for ex- 

 treme precision, volume units may be used, and we may con- 

 sider that a cubic centimeter of water weighs a gram. 



Obviously, the volume or weight of water lost from a certain 

 type of instrument for a unit of time may always be multiplied 

 by any value that the worker may like, so long as this value 

 is stated, and so long as it is always applied to all readings 

 from this same type of instrument. This treatment does not 

 alter the relative values of a series of comparable readings and 

 the results remain comparable. This principle makes it logi- 

 cal to use depth units instead of volume units, for free water 

 surfaces, for the depth of water lost from a given cylindrical 

 pan is the volume lost, multiplied by the reciprocal of the sur- 

 face area, this coefficient being a constant for the instrument. 



