TEMPERATURE OF THE SOIL. 21 



tinning until midwinter. As great a fall as 14° F. has been recorded in a 

 day. In January the temperature begins to rise, and the rise is gradual 

 until the last of March, when it becomes accelerated, so that by May the 

 soil approaches the temperature characteristic of early summer and is prob- 

 ably as great as that of the germinating period of midsummer. The course 

 of the temperature for the j^ear is fairly well illustrated by that of the 

 minima for 1908. From January 4 to March 29 the minimum lay between 

 50° and 60° F. ; between April 5 and April 26 it was between 60° and 70°; 

 in May the minimum was between 70° and 80°; in the first half of June 

 it was between 80° and 90°, and from then until July 1 5 the thermograph 

 did not record any temperature under 85°. With the coming of the sum- 

 mer rains the minimum began to fall and was near 80° until October 4; 

 it fell to about 60° until November 26. The minimum was about 50° until 

 December 14, and between 40° and 50° the remainder of the year. 



RECORD OF THERMOGRAPH: 30 CM. DEPTH. 



The soil temperatures for the depth of 30 cm. were recorded imder the 

 same conditions, except only the greater distance from the surface, as 

 those of the 15 cm. depth just described. The sensitive bulbs of both 

 instruments were only a meter apart. It needs but a glance at the records 

 of the two instruments to reveal their most striking difference : the daily 

 curve of the more deeply placed instrument has slighter undulations. In 

 addition, the annual variation of the two and the course of temperature 

 throughout the year are also markedly dissimilar. 



The usual daily range in temperature of the soil at a depth of 30 cm. is 

 about 2° F. ; the maximum daily range for 1908, which may be accepted 

 as approximately the maximum for the period under observation, was a 

 drop of 4° in July. But as this followed as a result of the rains it may be 

 taken to represent the vmusual rather than the usual temperature variation 

 for the depth. The maximum temperature for the entire period, 1905- 

 1909, occurred July 3. 1907, when the thermograph recorded 99° F. ; the 

 minimum was on January 3. 1907, at which time the temperature was 44°. 

 The annual extremes in temperature are presented in table 3. 



T.^BLE 3. — Maximum and Minimum Soil Temperatures at a Depth of 30 cm. 



