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 ordinary 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 



