From this table we can pass directly from the ratio of the ranges ;' at 

 the two depths to the corresponding value of the diffusivity K : — 



Results from Earnscleugh show that the ratio of the mean daily 

 ranges at the two depths was very nearly constant for each month of 

 the period, the lowest value of r being 2-2 and the highest being 2-6. 

 The average value of r as found from the mean daily ranges for each 

 month of this period is approximately 2-4, and from Table 4 this value 

 corresponds to a value of K equal to approximately 1-2 x 10"^ C.G.S. 

 units. This value seems abnormally low, and is, in fact, about one-quarter 

 of that found by Johnson and Davies from a consideration of the ratio 

 of the diurnal ranges observed at depths between 1 in. and 12 in. in grass 

 covered chalky soil. 



There is, however, an alternative method of determining the 

 diffusivity of the soil — that is, by means of the change of phase under- 

 gone by the temperature wave with increasing depth. The most 

 convenient way of measuring this phase-change in practice is to note 

 the time-lag between the times of maximum temperature at two specified 

 depths. If L is the lag in seconds, K is readily computed from the 



formula : K = —^ — I ' where as before T is the period (twenty-four 



hours or 8-64 x 10* seconds), and z^ and z.j are the two depths. 



For the case in which z^ — ^2 =2" Table 5 is computed from the 

 above formula and gives the calculated value of K for a range of values 

 of L. 



Table 5 



Tabulation of a considerable number of cases giving the times of 

 maximum temperature at the depths of 3 in. and 5 in. and also the 

 corresponding lag between the two depths shows that although there 

 is a certain variation among the times of maximum temperature at 

 the two depths the lags are fairly consistent. The mean of all the obser- 

 vations gives the time of maximum at 5 in. as 16-4 hours local time, 

 and the time of maximum at 3 in. as 14.9 hours local time. The average 

 lag is thus T5 hours for a distance of 2 in. in the soil. 



53 



