36 BULLETIN" 1059, U. S. DEPARTMENT OF AGRICULTURE. 



tissue paper, by sealing to the thermometer an empty cap or vial, 

 or by sealing on a vial filled with alcohol. Covering the bulb, of 

 course, retards the movement of the thermometer which changes of 

 soil temperature, but this is unimportant as compared with the sud- 

 den changes which would result from bringing the naked ther- 

 mometer up into the air. The thermometer to be used in these tubes 

 should be the Weather Bureau "mercurial thermometer," and may 

 be kept attached to the aluminum frame which affords much needed 

 protection. 



The use of registering maximum and minimum thermometers in 

 soil temperature work is not very satisfactory. It is true that the 

 standard Weather Bureau types of these instruments may be used on 

 the surface of the soil almost as well as in the air. The following 

 precautions, however, should be observed : 



1. To bring the thermometer into close contact with the soil and 

 to avoid unnecessary conduction the metal frame should be dis- 

 carded. 



2. The minimum registering thermometer should be protected from 

 insulation in the middle of the day, since such thermometers ordi- 

 narily will not bear temperatures in excess of 120° F. Also, there is 

 some tendency to distill the spirits and break the spirit column at 

 high temperatures. 



3. The thermometers must be nearly level. 



Maximum and minimum thermometers of the ordinary type are 

 not feasible at any depth, because they can not be kept level. 



A maximum thermometer may, however, be used in a vertical posi- 

 tion at any depth, provided the stricture of the capillary tube is suffi- 

 ciently close to carry the weight of the mercury above it. This is 

 technically almost impossible to accomplish, but one in a dozen maxi- 

 mum thermometers may serve the purpose. 



To use the registering minimum thermometer at any depth, it is 

 necessary that the stem be bent at a distance from the bulb approxi- 

 mately equal to the contemplated depth, and that the scale fall en- 

 tirely in that part of the stem above the bend, which is to be hori- 

 zontal. There is, of course, a limit beyond which this form of con- 

 struction can not be safely carried, since the alcohol in the stem, as 

 well as in the bulb, reflects temperature. An additional difficulty is 

 in the distillation that has been mentioned, but this may be largely 

 overcome by sufficiently high air pressure above the spirit column. 



For permanent stations the use of the telethermoscope or electric- 

 resistance thermometer may, in some cases, be advisable; but this 

 apparatus is expensive and delicate, can not be installed except with 

 considerable disturbance of the soil, and is subject to serious errors 

 if, for example, the batteries become weak or the galvanometer is not 

 perfectly leveled. Especially where great precision is necessary, as 



