1919-20.] Effect of Weather Changes on Soil Temperatures. 57 
to attempt to give a review of all the changes in underground temperatures 
brought about by weather changes at the surface. 
In, comparison with the variation of surface temperature, the regular 
pulsations of temperature underground follow well-known laws for ampli- 
tude and retardation according to depth ; but in these regular pulsations 
there are minor fluctuations which occur either day by day, or at irregular 
intervals, according to the weather and the state of the surface soil. 
Of these minor fluctuations the most important are due to the 
percolation of rain, the movements of soil air and moisture, the presence 
of a dry surface mulch, the prevalence of strong winds of low relative 
humidity, or the occurrence of frost and snow. We have thus several 
agencies at work tending to disturb the regular changes of temperature 
underground, and I propose to deal with each of these in turn in the 
light of my observations with electrical resistance thermometers during 
the past year. These observations have been kept in graphical form, 
and it is thus a simple matter to note the combined effect of the various 
agencies at work, and it is possible in favourable circumstances to assign 
to each agency its own share in the fluctuations. 
For a discussion such as this, mean daily values of underground 
temperature would be useless, as frequently the agency at work causing 
the fluctuations from the normal is of a temporary nature, and its effect 
would only come to view in hourly values during the period of its activity ; 
the whole of this paper is therefore based on the records of hourly values 
taken during the day and on selected nights during 1919. 
Throughout this paper I have used the ratio of the ranges of temperature 
at the 4-inch depth and at the surface as m y standard; this is a 
purely arbitrary choice, as the ratio of the ranges at the 8-inch depth and 
at the surface f^) would have served the same purpose, as similar 
VJ V 
R 
fluctuations — though in a less degree — are shown by the values of ^ s - 
R 0 
Below the 8-inch depth comparison is not possible, as the soil at that 
depth in the garden where the observations were made is a stony subsoil of 
quite a different nature to the surface soil. 
Section II. — Sunshine and Shade. 
(a) Movements of Soil Air and Water. 
It is an undoubted fact that changes in temperature* of the surface 
soil bring about considerable movements of the soil air ; these air move- 
