[CALLENDAR] OBSERVATIONS OF SOIL TEMPERATURES 67 
The weather continued bright and sunny till the 22nd. The isother- 
mal of 35° F. disappeared early on the 21st, and that of 40° F. penetrated 
to a depth of 20 inches by the 23rd. The rain of April 22nd was fol- 
lowed by several dull, cold days, without sun, which produced a marked 
retardation ; but with the beginning of May summer began with a 
phenomenal wave of heat. By May 4th the isothermal of 40° F, had 
disappeared, and the buds of nearly all the trees showed signs of burst- 
ing into leaf. The higher isothermals crowd into the ground very 
rapidly. By May 10th the isothermal of 60° F. had penetrated to a 
depth of 10 inches, and that of 50° F. to a depth of nearly 30 inches. 
The thermometer at a depth of 5 feet 6 inches had risen 4° F., but that 
at 9 feet, in the blue clay, remained within a tenth of a degree of 
41:5° F., showing no variation since April 17th. 
EFFECT oF VARIOUS CONDITIONS ON Sor, TEMPERATURES. 
The foregoing observations, taken in conjunction with the isother- 
mal diagram which they are intended to describe, illustrate very clearly 
a number of points in connection with changes of underground tempera- 
tures. Many of the results obtained are obvious, upon consideration, and 
have been observed previously ; but some, perhaps, of the conditions 
affecting soil temperatures have not been before observed under equally 
favourable circumstances. 
The protective effect of the surface covering of snow is very strongly 
marked. The thermometer lying undisturbed on the surface of the grass 
beneath the snow, afforded a means of observing the degree of protection 
more certain than has hitherto been obtained; and the results would 
appear to indicate that the protective effect is even more remarkable and 
important than the majority of meteorologists have been inclined to 
admit. 
The effect of heavy rainfall in diffusing heat and equalizing tem- 
perature in porous soils, is well illustrated by some of the observations. 
The importance of this factor has been frequently shown, but it has also 
been frequently overlooked in discussions of the thermal diffusivities of 
various soils. It would, doubtless, be possible from a more continued 
series of observations to deduce the effect in terns of the rainfall; and 
also to determine changes of level of underground water, in many cases, 
by observing changes of temperature and of diffusivity of the soil; but 
this would require a somewhat larger number of thermometers. 
The depth of frozen soil varies considerably in different years, accord- 
ing to the date and depth of the earlier falls of snow. By the aid of 
buried thermometers, it is easy to determine the depth to which the frost 
has penetrated at any time; it is also possible to observe the rate at 
which the ground thaws with considerable accuracy, and to predict with 
some certainty the date at which the ground will become finally thawed 
