129 
from mechanical injury by being led through a groove in a board. 
A narrow trench two meters long and about 20 cm. indepth was 
then dug, and the excavation was continued for a distance of 
half a meter more in a small tunnel. The tube was now laid in 
the trench and the bulb pushed to the farther end of the tunnel. 
This allowed the bulb to lie in contact with the undisturbed earth 
around the roots of the grasses and other plants growing there, 
and at a distance of 30 cm. (1 ft.) from the surface of the soil. 
The trench and tunnel were carefully packed with earth. The 
above arrangement places the bulb at a distance of 3.5 meters 
from the recording apparatus and gives a record of the tempera- 
ture of the soil in‘a fairly natural condition with respect to its 
porosity and permeability to air and water. The soil was clayey 
with a mixture of a small proportion of loam. 
Previously to the installation the two thermographs were 
placed in a closed room with a standardized mercurial ther- 
mometer (No. 2792 G. S. Reichsanstalt, Berlin) and were found 
to read approximately alike, allowing for a certain slowness of 
response in the thermographs (see tracings under date of April 
28, A and &, to AZ, Fig. 21). Both were put in place at the 
nursery on May 2 
The results obtained at this date are shown in Fig. 21, in 
which tracings of the thermographic records of the temperature 
of the air, and of the soil at the stated depth are given. The 
temperature of the soil is seen to exhibit two variations. In one 
series the lowest temperature is reached between 8 A. M. and 
noon and the highest between 8 P. M. and midnight. A second 
series of fluctuations follows the general variations in air tem- 
peratures as exemplified in “ warm waves’’ and ‘cold waves.” 
During the period of six weeks over which these records extend 
the heaviest rainfall of .65 inch appeared to exert no direct 
effect upon the temperature of the soil at the given depth, although 
the effect might have been masked by the general cooler tem- 
perature of the air following the rain. The highest temperature 
of the soif recorded was 14° C. (57° Fahr.) and the lowest 7° C. 
(44° Fahr.). It is too early to make generalizations upon the 
influence of the fluctuations of soil temperatures upon the growth 
