TEMPERATURE OF THE SOIL. 



The observations on the temperature of the soil have been confined to 

 two series, namely, to a continuous one by means of thermographs and to 

 a fairly large number of readings of thermometers . The thermometric read- 

 ings are for depths of i, 2, and 12 inches. The thermograph records date 

 from the summer of 1905 and the spring of the following year, and give the 

 temperature for one station, close by the laboratory building, on the north 

 slope of Tumamoc Hill, and for two depths, 15 and 30 cm. 



RECORD OF THERMOGRAPH: 15 CM. DEPTH. 



An almost continuous record of the temperature of the soil at the 15 

 cm. depth is at hand for the years 1905, 1906, 1907, 1908, and 1909. An 

 examination of these temperature graphs shows interesting features, some 

 of which will be noted. 



The record shows an undulating line of which the curve-crests correspond 

 to the warmest for each day, and the depressions the coldest. The crests 

 for any record (each one is for 7 days) are remarkably uniform in height, as 

 also the depressions are uniform in depth. The difference between the 

 crests and the depressions is about 8°F., with 12° as the greatest variation, 

 which usually occurs in March and July. Owing to the lagging of the soil 

 temperatures, as compared wdth those of the air, the maximum at this 

 depth is not attained until about 6 p. m. and the minimum about midnight. 



The uniformity of the daily temperature range is broken during stormy 

 periods, particularly in the winter season. In summer, the undulating 

 curve of daily variation, even during stormy periods, may be identified, 

 although its amplitude is greatly decreased. The extreme yearly range of 

 temperature for this depth, for any year since the records began, is 69° F. ; 

 and the extreme range for the entire period, 1905 to 1909, is 73° F. The 

 following table gives the greatest range in temperature for each year. 



Table 2. — Maximum and Minimum Soil Temperatures at a Depth of 15 cm. 



If we follow the course of the temperature record throughout the year, 

 we shall see that the period of greatest heat is during the latter part of 

 July, or immediately preceding the rainy season of midsummer. With the 

 advent of the rains, the temperatures suddenly decline, the decline con- 



