120 THE S TUDY OF PLANT COMMUNITIES * Chapter VI 



levers to a pen, which records on a graduated sheet revolving on a 

 drum. Both air and soil thermographs are used and are also obtain- 

 able in the same instrument, thus giving parallel records. If cost of 

 these instruments prohibits their use where a comparative study 

 of numerous stations is to be made, maximum and minimum ther- 

 mometers are the simplest solution. Placed in pairs and read and 



FIG. 62. Soil-air thermograph, which records the temperatures of soil and 

 air continuously on a revolving drum. The cable at right is about six feet 

 long and terminates in a sensitive bulb (not shown), which can be placed at 

 any level in the soil— Courtesy Friez Instrument Division, Bendix Aviation 

 Corporation. 



reset at regular intervals, they give the useful values of maximum 

 and minimum temperatures for the period of exposure. Above the 

 reservoir in a maximum thermometer is a constriction through 

 which, because of the force and volume of mercury involved, ex- 

 pansion easily forces the liquid. Contraction, however, develops 

 no pressure above the constriction, and the capillary column re- 

 mains essentially at the level of its highest rise. As with a clinical 

 thermometer, the column must be shaken or spun back down to 

 the reservoir when a new reading is desired. The minimum ther- 

 mometer has a small marker in its liquid, which, because of surface 

 tension at the top of the column, is pulled down as the tempera- 

 ture is lowered but is not raised with increasing temperature. Tilt- 

 ing the thermometer will immediately bring the marker back to 

 the top of the column in a new setting position. 



