74 JOHN W. SHIVE AND WILLIAM H. MARTIN 



influence the time of color change over the standard water sur- 

 face, is the temperature. 



For each of the hygrometric paper sHps here used, ten tests 

 of the time response were made in the laboratory, all at approxi- 

 mately the same temperature. The air temperature in the im- 

 mediate vicinity of the standard water surface and that of the 

 standard water surface at the time when the tests were made, 

 were here considered to be the same. The average time re- 

 sponse for each sUp, for the temperature at which the tests were 

 made, was calculated, and the value thus obtained was used 

 in the calculation of the time response of the slip in question 

 (over the standard water surface) at field temperatures. In the 

 standardization of the composite slips, all the precautions sug- 

 gested by Livingston and Shreve were cao-efully observed, so 

 that the calculations of the time response over the standard 

 evaporating surface at field temperatures should be fully as 

 satisfactory as actual field tests themsleves. It has been asserted 

 bj^ the last-named authors'^ that "it is practically certain that 

 calculations for field temperatures, made from the average of 

 a large number of tests in the laboratory, will prove to be more 

 nearly correct than most single field tests, or even averages of 

 a number of the latter." 



The time required for the color change of the hygrometric 

 paper from the dark blue to the light blue of the two color stand- 

 ards, upoji the leaf surface, was determined by means of a stop 

 watch. The time period in question will be designated by T?, 

 and the time period for the corresponding color change over the 

 standard water surface will be termed T^, following a similar 

 usage of these symbols by Trelease and Livingston. The ratio 

 of the calculated time period (TJ required for the color change 

 over the standard evaporating surface at field temperature, to 

 the time period (T^) required for the corresponding color change 

 over the leaf at the same temperature, has been called by Liv- 

 ingston the index of transpiring power of the given leaf surface. 



T 

 This index, ~, is a measure of the relative capacity of the leaf 



I; 



" Livingston, B. E. and Shreve, Edith B., 1916, p. 303, I. c. 



