66 THE HABITAT 
the liquid expands. This is due to the fact that the fluid expands too 
slowly to carry the marker upward, while the surface tension causes it to 
be drawn downward as the fluid contracts. The minimum temperature 
is indicated by the upper end of the marker. In setting up the thermome- 
ters, they are attached by special thumbscrews to a support which holds 
. them in an oblique position. The minimum is placed in a special holder 
above the maxi- 
mum which rests 
on a pin that is 
used also for 
screwing the piv- 
ot-screw into po- 
: ; sition. The sup- 
Fig. 17. Maximum-minimum thermometer. port is screwed 
tightly to the cross-piece of a post, or in forest formations it is fastened di- 
rectly to a board nailed upon a tree trunk. A shelter has not been used in 
ecological work, although it is the rule in meteorological observations. The 
minimum thermometer is set for registering by raising the free end, so 
that the marker runs to the end of the column. The mercury of the maxi- 
mum is driven back into the bulb by whirling it rapidly on the pivot-screw 
after the pin has been taken out. This must be done with care in order 
that the bulb may not be- 
broken. As soon as the in- 
strument comes to rest, it 
is raised and the pin re- 
placed, great care being 
taken to lift it no higher 
than is necessary. When 
the night maximum is 
sought, the thermometer 
should be whirled several 
times in order to drive the Fig. 18. Terrestrial radiation thermometer. 
column sufficiently low. 
Usually, in such cases, a record is made of this point to make sure that 
the maximum read is the actual one. If the pivot-screw is kept well oiled, . 
less force will be required to drive the mercury back. In practice, the 
thermometers have been observed at 6:00 a.m. and 6:00 P.M. each day, 
thus permitting the reading of the maximum-minimum for both day and 
night. Pairs of maximum-minimum thermometers are to be obtained from 
H. J. Green, 1191 Bedford Ave., Brooklyn, or Julien P. Friez, Baltimore, 
Maryland, at a cost of $8.25. 
