TEMPERATURE 



65 



gravel soils. Repeated tests of this have been made by simultaneous read- 

 ings in permanent and newly made holes, and the results have always been 

 the same. It has even been found that the error is usually less than i degree 

 when the hole is left uncovered, if it is more than 9 inches deep. A slight 

 source of error lies in the fact that the thermometer must be raised to make 

 the reading. With a little practice, however, the top of the 

 column of mercury may be raised to the surface and read be- 

 fore the change of temperature can react upon it. This is 

 especially important in very moist or wet soils where the bulb 

 becomes coated with a film of moisture. This evaporates 

 when the bulb is brought into the air, and after a moment or 

 two the mercury slowly falls. 



Regular soil thermometers are indispensable when read- 

 ings are desired at depths greater than 12-18 inches. They 

 possess several disadvantages which restrict their use almost 

 wholly to permanent stations. It is scarcely possible to carry 

 them on field trips, and the time required to place them in 

 the soil renders them practically useless for single readings. 

 Moreover, the instruments are expensive, ranging in price 

 from $7 for the two-foot thermometer, to $19 for the eight- 

 foot one. When it is recognized that deep-seated tempera- 

 tures are extremely constant and that the slight fluctuations 

 affect, as a rule, only the relatively stable shrubs and trees, 

 it is evident that such temperatures are of restricted impor- 

 tance. Still, in any habitat, they must be ascertained before 

 they can well be ignored, though it is unwise to spend much 

 time and energy in their determination. Soil thermometers 

 of the form illustrated may be obtained from H. J. Green, 

 Brooklvn. 



Fig. 16. Soil 

 thermometer 



96. Maximum-minimum tliermometers. These are used 

 for determining the range of temperature within a given 

 period, usually a day. Since they are much cheaper than 

 thermographs, they can replace these in part, although they 

 merely indicate the maximum and minimum temperatures for 

 the day, and do not register the time when each occurs. The 

 maximum is a mercurial thermometer with a constriction in the tube just 

 above the bulb ; this allows the mercury to pass out as it expands, but 

 prevents it from, running back, thus registering the maximum temperature. 

 The minimum thermometer contains alcohol. The column carries a tiny 

 dumbbell-shaped marker which moves down with it, but will not rise as 



