EXPERIMENT STATION BULLETINS. 813 



average temperature was raised to the sarae magnitude as that of the 

 sand and gravel. 



There are good reasons to believe, that a cover of growing vegetation 

 would cause identical equality of average temperature in the diverse 

 types of soil as the cover of sand. Hence, variations in temperature that 

 are observed in these soils in bare condition, may not exist when they 

 are under crops, i. e., after complete thawing has taken place. 



The remarkably even magnitude of average temperature of the different 

 types of soil, both when their surface was covered and uncovered, in the 

 fall and winter months, is of the greatest interest. It shows that there 

 was no one single factor during these seasons, not even the specific heat 

 of water, which had a predominant and controlling influence in causing 

 these various soils to cool differently and be unequally warm. 



The rapidity with which the clay and loam thawed and the rise of 

 their temperature to the same degree as that of sand and gravel in such 

 a short time, both when their surface was covered and not covered with 

 sand, contradicts the exaggerated general belief that these soils possess 

 a greatly lower temperature during the spring and therefore should be 

 classed as cold soils. This last term, however, may be justly applied 

 to peat, especially when it is not covered with sand. 



There was some heat coming to the surface during the winter from the 

 lower and warmer depths which tended to keep the temperature of the 

 upper depths higher, and aided in the thawing and raising the tempera- 

 ture in the spring. The average air temperature was lower than that of 

 any soil at the 2, 4, 6, 12 and 18-inch depths, throughout the year. The 

 amplitude of the air temperature, however, was far greater than that of 

 all the soils at any of the above depths, also throughout the year. 



The maximum temperature of all the soils at one quarter of an inch 

 depth was about 30 °F higher during hot and clear days than that of the 

 air at an elevation of four feet. 



The minimum temperature of all the soils except peat, however, imme- 

 diately at the surface was only about 1 or 2°F higher, as a monthly aver- 

 age, than that of the air at a height of four feet. That of the peat 

 was slightly lower than that of the air. 



Unless the various soils were frozen they always had a gradient of 

 temperature at their adjacent depths. This gradient of temperature, 

 however, reversed itself between day and night during the warm part of 

 the year to the depth that the diurnal-nocturnal amplitude of oscillation 

 of temperature extended. During the day it decreased from the surface 

 downward and during the night the reverse was true. There were two 

 periods, one in the morning and one in the evening, when the reversion 

 took place, that this gradient was not regular. During the day, at the 

 time that the maximum temperature was attained at the surface, this 

 gradient was very large at the adjacent dephts, the difference between 

 the surface and the Gin. depth being sometimes as high as 30° F, but 

 during the night at the time when the minimum was reached at the 

 surface, it was comparatively small, the greatest differences between the 

 same depths being only about 10°F. 



The soils, down to the depth that the diurnal-nocturnal amplitude of 

 oscillation of temperature extended, received their temperature in the 

 form of waves. There were, of course, two types of waves, a warm and a 



