i6o 
Journal of Agricultural Research 
Vol. V, No. 4 
up for this decrease, so that an equilibrium of pressure will exist, is 
mainly from the outside atmosphere. After a certain depth is reached, 
the maximum temperature wave entirely disappears, and there is no 
more upward expulsion or movement of air. From now on, as the 
temperature of the soil is further decreased and the volume of its air 
diminished correspondingly, the current of flow of air into the soil is 
entirely from the outside atmosphere. This downward flow of air will 
continue until the soil temperature begins to rise again and the cycle 
recommences. When the minimum temperature of the surface soil is 
reached, it is, as a rule, about the same or slightly higher than that of 
the air immediately above. The temperature of the air at about this 
period increases with the height in the same manner as the temperature 
of the soil increases with depth, which is just the opposite from what it 
is during the day. This increase instead of decrease of temperature at 
night with a rise in elevation is called “surface temperature inversion/' 
At this minimum temperature the atmospheric pressure approaches its 
maximum, and the inward flow of air is thereby facilitated. 
All the foregoing facts lead to the enunciation of a general law that 
during the day, as the temperature rises, the soil air tends to flow out¬ 
ward into the atmosphere, and during the night, as the temperature falls, 
air from the atmosphere tends to flow inward into the soil. This law 
diametrically opposes the prevalent theory that during the night there is 
an upward movement of moist warm air. The above law, however, 
seems to be borne out by logic and appears to be confirmed by experi¬ 
mental evidence subsequently to be presented. The prevalent theory 
seems unreasonable; for instance, if it is admitted, which it must be, 
that the soil air escapes into the atmosphere during the day as the tem¬ 
perature rises, then where and when does the soil obtain its air if it con¬ 
tinues to give up air even during the night ? It might be argued that it 
is vapor that is rising to the surface and not air. That is inconceivable 
in the present case. It is true that distillation would occur if the ampli¬ 
tude of temperature were appreciable and constant, but it has been shown 
that the temperature of the whole column of soil decreases constantly and 
that an air current from the cold atmosphere is drawn inward which tends 
to encounter and oppose any upward movement of vapor rising from 
any difference in temperature. Moreover, granting for sake of argument 
that there is a vapor rising from the warmer soil to the colder soil at the 
surface, the amount would be extremely small—too small to account for 
the great quantity of dew commonly noted—because the temperature 
amplitudes of the soil at different depths at night are never very great. 
In fact, during the spring months, as the temperature of the lower depths 
continually rises and the trend of the air temperature is upward, the 
range of temperature between the surface and the lower depths, say 6 
inches, is small, usually amounting only to about 2° or 3 0 C. The greatest 
differences in temperature at the different depths in the morning occur 
