The velocity of propagation of temperature waves will be the same under similar conditions in two substances which possess the same **diffusivity**, although they may differ in conductivity.

No calorimetric observations are required, but the results are obtained in terms of the thermal capacity of unit volume c, and the measurements give the **diffusivity** klc, instead of the calorimetric conductivity k.

The method of deducing the **diffusivity** from these curves is as follows: - The total quantity of heat absorbed by the soil per unit area of surface between any two dates, and any two depths, x' and x", is equal to c times the area included between the corresponding curves.

We thus obtain the simple equation k'(de'/dx') - k"(de"/dx") =c (area between curves)/(T - T'), (4) by means of which the average value of the **diffusivity** klc can be found for any convenient interval of time, at different seasons of the year, in different states of the soil.

For the particular soil in question it was found that the **diffusivity** varied enormously with the degree of moisture, falling as low as ooio C.G.S.

On some occasions, owing to the sudden melting of a surface layer of ice and snow, a large quantity of cold water, percolating rapidly, gave for a short time values of the **diffusivity** as high as.

Excluding these exceptional cases, however, the variations of the **diffusivity** appeared to follow the variations of the seasons with considerable regularity in successive years.

The **diffusivity** can be deduced from observations at different depths x' and x", by observing the ratio of the amplitudes, which is (x '- x ") for a simple-harmonic wave.

- **Diffusivity** of Sandy Soils.

Rambaut's results were obtained with similar instruments similarly located, but he did not investigate the seasonal variations of **diffusivity**, or the effect of percolation.

Angstrom's Method consists in observing the propagation of heat waves in a bar, and is probably the most accurate method for 4 4 thehi 's ' 'so ' d 60 measuring the **diffusivity** of a metal, since the conditions may be widely varied and the correction for external loss of heat can be made comparatively small.

The extreme divergence of the resulting values of the **diffusivity**, including eight independent series of measurements on different days, was less than I %.