68 THE HABITAT 
are communicated to a hand carrying a pen. The latter traces a line on-the 
record.sheet which is attached to a metal disk made to revolve by ari eight- 
day clock. In practice the thermograph is set up in the shelter which con- 
tains the psychrograph, and in exactly the same manner. The clock is 
wound, the record put in place, and the.pen inked in the same way: also. 
The proper position of the pen is determined by making a careful ther- 
mometer reading under the 
shelter, and then regulat- 
ing the pen-hand by means 
of the screws at the base 
of it. A similar test read- 
ing is also made each 
week, when the clock is re- 
|} wound. A_ record sheet 
‘ may be left in position for 
three weeks, the pen being 
filled each week with a dif- 
ferent ink. The: fixed or- 
der of using the inks is red, 
blue, and green as already 
indicated. 
Owing to the fact that 
‘they are practically station- 
ary, soil thermographs are 
of slight value, except at 
base stations. Here, the 
facts that they are expen- 
sive, that the soil-tempera- 
tures are of relatively lit- 
Fig. 20. Shelter for thermograph. tle importance, and that 
-they can be determined as 
easily, or nearly so, by simple thermometers, make the use of such instru- 
ments altogether unnecessary, if not, indeed, undesirable. In a perfectly 
equipped research station, they undoubtedly have their use, but at ordinary 
stations, and in the case of private investigators, their value is in no wise 
commensurate with their cost. 
