66 ROYAL SOCIETY OF CANADA 
During the next fortnight, which was dry, the fall went on steadily 
but slowly. The depth of partially frozen soil increased to 9 inches. 
The mean value deduced for the thermal diffusivity of the soil, from the 
observations between 20 and 66 inches, for the week December 2nd to 
December 9th, is ‘0065 in C. G. S. measure. 
The most remarkable and sudden fall of soil temperature through- 
out the winter began on December 12th, and was caused by a fall of 
rain, which melted two or three inches of snow, and percolated rapidly 
through the dry and partially frozen sandy soil to the level of the blue 
clay beneath. This fall differs entirely in character from that due to 
thermal diffusion, in being very rapid and nearly simultaneous at all 
depths. It was followed by a partial recovery in the upper layers of 
soil, due to thermal diffusion from beneath. 
The next fortnight was dry, and the latter half cold. The ground 
being free from snow, became thoroughly frozen to a depth of 13 inches. 
The first heavy snowfall (about 1 foot) occurred on December 27th 
from which date the ground remained covered with snow till April 11th. 
Such was the protective effect of the snow covering, that although the 
mean temperature of the next two days was 10° below zero, the cold did 
not penetrate any further into the soil. By the end of the next fort- 
night, in spite of a mean air temperature of 10° F., the isothermal of 
30° F. had disappeared from the soil, and did not again reappear through- 
out the winter. 
From this date onwards the depth of frozen ground slowly but 
steadily diminished, The upper thermometers remained nearly station 
ary. The lower thermometers fell slowly, with slight fluctuations, due 
probably to movements of underground water. 
The final melting of the snow between April 7th and 14th was 
accompanied by heavy falls of rain. The water could not penetrate 
directly through the frozen ground, but found its way into the subsoil 
by devious routes. The effect was to produce a sudden fall in all the 
lower thermometers, the upper ones remaining unaffected. It may be 
noticed that on this occasion the deepest thermometer, in the blue clay, 
was the first to be affected, whereas on the previous occasion, December 
13th, when the water percolated directly through from above, the ther- 
mometer in the blue clay was scarcely affected at all. 
The final thawing of the ground took place at a depth of 10 inches 
on April 19th. The 10 inch thermometer, which had remained within 
less than a tenth of a degree of 32° F. for about two months previously, 
showed a depression of three-tenths of a degree when the thaw reached 
it; probably owing to the lowering of the melting point by salts dis- 
solved in the accumulated surface water. The percolation of this water 
caused a slight simultaneous fall in the two thermometers next below. 
