ATMOMETRY AND THE ATMOMETER 69 



tion, upon a rotating table, 18 either indoors or in the open — in 

 the latter case, without rain. Where radio-atmometers (which 

 receive attention below) are employed, sunshine must be ex- 

 cluded, as well as any other form of strong radiant energy. To 

 accelerate evaporation, and thus give larger readings for shorter 

 time periods, an electric fan is frequently so placed as to pass a 

 strong current of air across the rotating table. The results are 

 the same, aside from the time required, whether the fan is em- 

 ployed or not. 



The rotating table now in use in the Laboratory of Plant 

 Physiology of the Johns Hopkins University is somewhat over 

 2 m. in diameter and rotates two or three times per minute. 

 At the Desert Laboratory Mr. J. W. Shive constructed, in the 

 summer of 1913, a rather similar but smaller table, about 130 

 cm. in diameter, built upon a single bicycle wheel in a manner 

 somewhat similar to that adopted by Nichols. 19 It makes no 

 difference, as far as these matters have been studied, what size 

 of table is employed or at what speed it rotates. 



Of course the purpose of the rotation is to give the various 

 cups equal treatment; since it is practically impossible to main- 

 tain exactly similar or even constant air conditions throughout 

 a group of stationary cups, this is not attempted, but each in- 

 dividual cup is made to march through the same series of condi- 

 tions as does every other cup of the group. The principle of this 

 procedure is related to that upon which is based the long familiar 

 use of clinostats, to equalize one-sided influences, as these affect 

 growing plants. 



The cups may be mounted, for operation upon the rotating 

 table, in any convenient manner, usually on small-mouth bottles 

 or upon burettes (see the following section on installation), but 

 it is to be borne in mind that every cup should receive the same 

 influence due to the trailing vapor blanket emanating from the 

 next preceding one in the series. This means that all cups 

 should be at the same elevation from the table and that they 

 should be equally spaced in the same circle. 



18 Livingston, loc. cit., 1912. 



19 Nichols, G. N., A simple revolving table for standardizing porous cup at- 

 mometers. Bot. Caz. 56: 148-152, 1913. 



