158 BURTON EDWARD LIVINGSTON 



work in transpiration and climatology. The only critical points 

 to be considered in connection with work in the open have to do 

 with the questions whether or not the platform chosen for the 

 tests is freely exposed to wind movement in all directions and 

 whether the air moving over it is as free from dust as is possible. 



But weather conditions so frequently preclude the standardiza- 

 tion of these instruments out of doors, especially in any but the 

 most rainless climates, that the indoor method has been resorted to 

 in many instances. All cups used as standards at Tucson, and 

 all standardized cups furnished to other workers are now standard- 

 ized within doors, at the Laboratory of Plant Physiology of the 

 Johns Hopkins University. 



Since the relative importance of convection currents in the air 

 about the cups is very greatly magnified as soon as the general 

 sweeping action of the horizontal air movement (which is so 

 nearly always effective out of doors) is checked, the problem of 

 the equalization of such currents is of prime importance for indoor 

 work. Also, especially in artificially heated rooms, there is a 

 high degree of probability that, during any period Of observation, 

 one portion of the room may be warmer or cooler than another. 

 Also, it is desirable to increase the rate of evaporation in the house 

 by the use of an electric fan, a procedure which could not possibly 

 furnish the same air movement to all of a series of stationary cups. 



It is practically impossible to place two or more instruments 

 in a room so that they will all be exposed to the same conditions 

 of temperature, humidity and air movement. The problem had 

 to be attacked from the other side, it was necessary to devise 

 means by which, with various parts of the room exhibiting 

 various aerial conditions, all cups should nevertheless be subjected 

 to exactly the same series of changes during any time period. This 

 has been accomplished by the use of a rotating table, near the outer 

 margin of which the instruments (in their bottles) are placed. As 

 the table rotates, one cup after another marches into and out of 

 any given region of the surrounding air, and, with every rotation, 

 each cup returns rhythmically to this same region. The move- 

 ment of the cups tends to wipe off the vapor blanket which would 

 hang over each one were it stationary, and, if higher rates of evap- 



