26 BULLETIN 1059, U. S. DEPARTMENT OF AGRICULTURE. 



Recording instruments : 



Thermograph, complete, with a year's supply of blank 

 forms, pen and ink .$70. 00 



Combined air and soil (or water) thermograph, complete, 

 with bulb and connecting tube 10 feet long ; a year's sup- 

 of blank forms 105.00 



Extra length tube (above 10 feet) for above instru- 

 ment per foot . 50 



Thermograph, short range of temperature (probably duty- 

 free prices) 32.00 



Thermograph, large range of temperature (probably duty- 

 free prices) 42.00 



Recording thermometers, dial type, with one or two pens 



and bulbs $75. 00 to 150. 00 



SOIL TEMPERATURES. 



Soil temperatures are probably even more important in forest 

 study, especially when questions of initiation and distribution are 

 involved, than air temperatures. Opportunities for obtaining data 

 on the former will probably be more restricted, because of the greater 

 difficulty and expense of installing satisfactory apparatus. They are 

 at present measured at very few, if any, Weather Bureau stations. 



It should be strongly emphasized that the study of soil tempera- 

 tures is in a primitive stage, and that the devising of both instru- 

 ments and methods offers great opportunity for the investigator, 

 especially in the search for the exact, controlling conditions of the 

 soil's surface. The present discussion does not attempt to consider 

 all the special investigations which are undoubtedly needed, but con- 

 fines itself largely to routine methods, by which a broader survey of 

 soil temperature conditions may be gained, making possible regional 

 and site comparisons on something like a standard basis. 



Purposes to be Served. 



The number, and to a considerable extent, the method of soil tem- 

 perature observations to be made, will depend on the object. Some 

 of the purposes to be served may be summarized as follows : 



1. Rather general comparisons of temperature conditions in dif- 

 ferent plant formations and regions. For this purpose, soil tem- 

 peratures may have some advantage over air temperatures, in that 

 the former reflect to a considerable degree the amount of insolation 

 received at the ground ; and it must be admitted that air tempera- 

 tures, without radiation measurements, really give no indications of 

 the temperatures experienced by the plant. For this broad purpose, 

 temperatures at a depth of one foot are perhaps most satisfactory. 



2. Very careful comparisons of the extreme temperatures to which 

 plants are subjected on the various sites. There is much reason to 

 believe that maximum temperatures are often the forbidding factor 

 in the extension of the range of any given species and that they react 



