7@ ROYAL SOCIETY OF CANADA 
losing heat, especially at those periods of the year when the gain or loss 
is most rapid and important. This point is illustrated in Fig. 5, in which 
are shown together the curves obtained by plotting the daily observations 
of the 20-inch thermometer and the integrated curve of sunshine. It 
will be observed, on comparing the curves, that for every variation in the 
curve of total sunshine, starting from the date on which the ground was 
thawed, there is a variation in the temperature curve, at a depth of 20 
inches, similar to the former, but taking place about a day later. 
The temperature curve takes account of diminished loss of heat at 
night, as well as of increased absorption of heat by day. This is shown 
in many cases by slight differences between the curves, but most clearly 
by the rapid increase in steepness between the 4th and 10th of May, 
during which period the night radiation was checked by an almost con- 
tinuous haze of cirrhus clouds. 
The sunshine curve has been plotted by simply adding for each day 
the observed percentage of possible sunshine. The similarity of the curves 
would have been more striking if allowance had been made for the 
rapidly increasing altitude of the sun and length of the day. But there 
was not time to perform the reductions up to date. The effect of a dull 
day is nearly always shown to some extent in the reading of the 20-inch 
thermometer taken at noon on the following day, but the effect persists 
for two days. In some cases the effect on the second day is obscured by 
another hot day or by rainfall. 
Merxops OF DETERMINING THE THERMAL DIFFUSIVITY OF THE SOIL. 
The rate at which changes of temperature are propagated in any 
soil depends upon the value of a constant termed by Lord Kelvin the 
“ thermal diffusivity,” which is the ratio of the thermal conductivity # to 
the thermal capacity ¢ of unit volume. Much attention has been devoted 
by Lord Kelvin, Principal Forbes, Professor Everett, Angstrém, and 
other men of science to the determination of this important constant for 
various soils by observations with underground thermometers. This 
constant has also been determined by direct laboratory experiments for 
various rock and soil materials by other experimentalists, and particu- 
larly by a committee of the British Association.' 
The results with buried thermometers have generally been obtained 
by a comparison of the amplitudes and phases of the annual wave of 
temperature at different depths. As the committee point out, the values 
obtained by different observers for similar soils differ very widely among 
themselves, and do not agree as closely as might be expected with the 
results of direct experiment. This is, no doubt, partly to be explained 
by the difficulty of constructing accurate and suitable thermometers, 
which could be read without being disturbed. It would appear, how- 
2 
1 B. A. Reports, 1881, p. 126. 
