431 



the variable temperatures the temperature begins to rise at eight A. M., 

 reaches a maximum about two P. M., and falls during the following six 

 hours to a point near the average for the night. We made our observa- 

 tions the first thing in the morning and the last thing in the evening, 

 usually beginning at eight or earlier, and ending as late as six or six-thirty, 

 and sometimes seven, in the evening, when the experiments were gone 

 over twice a day. While carrying with it me possibility of a very slight 

 error in the total temperatures, any phenomenon occurring so as to be 

 first noted in the morning observation was recorded as having taken place 

 at S :00 A. M. Any phenomenon noted in the afternoon observation was 

 recorded as having taken place at 8 :U0 P. IM. With this division of mean 

 temperatures for half days, it was easily possible to compute the means 

 for any nimiber of clays with an adding machine, as a one-week period 

 would contain only fourteen items. The humidities were treated in a 

 similar fashion. The accounting was greatly simplified by this routine 

 clerical work, which proved to be on the whole very satisfactory, although 

 done by students who were paid very little. 



In experiments with very variable temperature, the sheets were given 

 a third type of inspection. The daily temperature and humidity cur^-es 

 were inspected, and notation was made of the night humidity and the 

 night temperature, which under most of our experimental conditions was 

 intentionally kept at a constant level. The hour at which this low level 

 was ordinarily reached in the evening and at which the temperature began 

 to rise in the morning was noted, and this temperature was called the base 

 temperature, as under the experimental conditions and often imder out- 

 door conditions the points marking this low level approach a straight line. 

 This base temperature had a corresponding base humidity. The base 

 temperature for each day was then noted by inspection and recorded on 

 a separate sheet, together with the absolute maximum and absolute mini- 

 mum and the amoimt of elevation above the base for each day. In Tables 

 XVIII, XIX, and XXVII these data are presented in full, for they proved 

 to be significant criteria of the climatic factors influencing the rate of 

 development. 



Standard Atmomctcrs. The atmometer used was the Livingston 

 porous cup atmometer, obtained from the Plant World, Tucson, Arizona. 

 The standardized cups ordinarily obtained, after use ranging from one to 

 three months, depending upon air conditions, were standardized. For 

 this purpose a wheel having a diameter of 38 inches was fastened in a 

 horizontal position on a table and turned at the rate of approximately 

 one revolution per second by a belt from a J4 h. p. motor making 1,200 

 R. P. M. The upper side of the wheel bore twelve upright posts, giving 

 it a capacity of twenty-four atmometers at one time, although only twelve 

 were commonly run at a time. These were standardized against a fresh 

 atmometer, and then scoured, emeried, and re-standardized, and used 

 until the standard fell to 0.50 or rose to 1.00, after which they were used 

 as irrigators in the chinch-bug work. It was desirable to have this piece 

 of apparatus on account of the large number of atmometers installed. 



