76 EXPERIMENT STATION. [Jan. 



b e and B E are assumed to have no effect on the germination 

 of the cress seeds. It is required first to determine the height 

 of I) e above c f, which height is assumed to be the same as the 

 height of B E above C F. 



According to our assumptions, the area of the figure abed 

 is equal to the area of the figure ABED. If we let the height 

 b c = B C = x ; and if we let the elapsed time in hours for 

 figure M represented by c f = y ; and let the elapsed time in 

 hours for figure N be represented by C F =: y' ; then allowing 

 m to represent the total sum temperature for the figure M = 

 (a c f d), and n to represent the total sum temperature above 

 zero recorded in the figure N (A C F D), we may form the fol- 

 lowing algebraic equation : — 



m — yx = n — y'x 



(y — y')x = vi — n 



x = 



As the quantities m, n, y and ?/ are all directly measurable 

 on any two thermograph records thus compared, x may be easily 

 computed in concrete numbers. 



Some difficulty arises in the use of this formula for deter- 

 mining the value of x, as when any single thermograph record 

 is compared successively with several others taken at ran- 

 dom, decidedly irregular results follow. Values for x can be 

 found varying all the way from — 1° to -f- 60°; and 1 hough 

 the majority of values lie between 5° and 10°, there is still too 

 great variation to make the result satisfactory. This compara- 

 tively great variation is due, however, not to any essential 

 inaccuracy in the method, but the smallness of the numbers 

 employed. 



In order to get rid of the relatively great variations shown 

 in individual comparisons and to find a reliable average for the 

 whole body of records, these records were plotted as shown in 

 Fig. .">. Here each dot shows the result of a single experiment, 

 referred to a horizontal axis for time and to a vertical axis for 

 accumulated temperature. The distribution of these dots dem- 

 onstrates at once the practically uniform character of the results. 



