[CALLENDAR A m'leod] OBSERVATIONS OF SOIL TEMPERATURES 115 



ent thermometers is so close that we think it must be partly accidental. 

 The uniformity of the soil, however, is such that samples of the sand from 

 différent depths cannot be distinguished, except in some cases by different 

 degrees of wetness. 



The depth-temperature curves during the winter months indicate by 

 the steeper gradient between 66 and 108 inches, a higher value of the 

 conductivity in the lower stratum. We may naturally sujjpose that this 

 is due to the greater wetness. During these months the diffusivity is very 

 low, but has also a higher value in the lower stratum. In the yearl}^ 

 average this small difference disappears owing to the much greater effect 

 of percolation at other Reasons. 



It will l)e observed that the yearly average obtained by the direct 

 method, is in fairly close agreement with that obtained from the annual 

 range. 



Ya LUES OF THE THERMAL DiFFUSIVITY AT DIFFERENT DatES. 



In the last year's communication, a direct method was described of 

 determining the thermal diffusivity of the soil in different conditions at 

 different seasons of the year. 



From the observations with the different thermometers at any date 

 a curve may be drawn showing the variation of temperatui-e at different 

 depths for that epoch. A series of such curves, take^i as samples from 

 the observations, are shown in Fig. II. The area inchided between any 

 two of these curves when multiplied by c, the thermal capacity of unit 

 volume, gives the total quantity of heat absorbed per unit area h y any 

 stratum of the soil between the dates and depths for which the curves 

 are taken. The quantity of heat per unit area which passes by conduc- 

 tion into any stratum of the soil at a depth x^ in any time, is equal 

 to the product of the thermal conductivity k and the mean temperature 



gradient — - at the depth x\ multiplied by t. the interval of time con- 

 sidered. The quantity of heat absorbed b}' a sti-atum of soil between 

 the depths x■^ and x^ is therefore equal to /." - y- — 1" r X f. 



The mean value of — > the temperature gradient at any depth, for any 



interval of time, is easily found by drawing tangents to the temperature- 

 depth curves. By equating the two expressions for the quantity of heat 



absorbed, the value of the diffusivity - is found. 



With the more complete series of observations at our disposal, we 

 have been able to considerably extend the apphcation of the method, and 

 to deduce some results of general interest for different seasons of the 

 year. A large number of typical depth-temperature curves have been 



