38 BULLETIN 1059, U. S. DEPARTMENT OF AGRICULTURE. 
is to indicate the temperature can not be expected to react to insola- 
tion quite as the soil does, nor to be exactly in temperature equilib- 
rium with the soil. 
Since the technique has not been well developed, the following 
suggestions are made in the hope of obtaining somewhat comparable 
results by different investigators. 
1. The bulb of the (maximum-registering) thermometer or the 
bulb of the thermograph should be exactly half buried, the object 
lying in a horizontal position. The lower surface may then be a 
little coler than the surface of the soil, but the exposed surface may 
be a little warmer. 
2. In order that the thermometer or thermograph bulb may have 
absorptive capacity for insolation similar to that of the soil studied, 
the exposed surface should be coated with linseed oil, and while this 
is still moist enough soil should be sprinkled upon it to form a thin 
coating. It may be necessary to repeat this at rather frequent in- 
tervals. 
3. The thermometer or thermograph should be disturbed as little 
as possible, since, if the soil about it is kept loose, it will not be 
normally moist and will not have the temperature of undisturbed 
soil. A maximum thermometer of the ordinary type must, of course, 
be raised for setting, so that for frequent comparisons of thermo- 
graph and thermometer, the ordinal cylindrical-bulb mercurial 
thermometer may be most satisfactory. 
4. Provision must be made for recording temperatures far in 
excess of those of the air or deeper soil. It will be safest to allow 
for an excess of full 100° F. over the 1-foot soil temperature, where- 
ever direct insolation is received during several hours of the day. 
Instruments. 
For mercurial thermometers, combined air-and-soil thermographs, 
and recording thermometers (equally adapted to air, soil, and water 
measurements) , see " Instruments " listed under "Air tempera- 
tures " : 
Special soil thermometers, wood encased, with stem long 
enough to be read above the surface of the ground, for 
depths of 6 inches to 3 feet $6.00 to $10.00 
Soil (or water) thermograph, with connecting tube, pens. 
ink, forms, etc. (bulb is about 1 inch by 12 inches) 85.00 
Soil thermograph 
Telethermoscope (electric resistance thermometer) with one 
bulb and nonrecording galvanometer 95.00 
Switch for 6 thermocouples (Galvanometer requires about 3 
dry cells) 16. 00 
