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- 

 port is screwed 



Maximum-minimum thermometer. 



Fig. 17. 



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 

 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. 



Fig. 18. Terrestrial radiation thermometer. 



