From Glossary of Meteorology
Two forms of the mercury-in-glass thermometer are used for this purpose. For measurement at small depths, a thermometer with a right-angle bend in the stem is used. The bulb is inserted into a hole in the ground with the stem lying along the surface. A thermometer that has been fused into an outer protecting glass shield is used for measurement at greater depths. Wax is inserted between the bulb and the shield to increase the time constant. To obtain a measurement, the instrument is lowered into a steel tube that has been driven into the soil to the desired depth.