TEMPERATURE 67 
97. Radiation thermometers. These are used to determine the radiation 
in the air, and from the soil, i. e., for solar and terrestrial radiation. The 
latter alone has been employed in the study of habitats, chiefly for the 
purpose of ascertaining the difference in the cooling of different soils at 
night. The terrestrial radiation thermometer is merely a special form of 
minimum thermometer, so 
arranged in a support that 
the bulb can be placed di- 
rectly above the soil or plant 
to be studied. It is other- 
wise operated exactly like 
the minimum thermometer, 
and the reading gives the 
minimum temperature which 
the air above the plant or 
soil reaches, not the amount 
of radiation. As a conse- 
quence, these instruments 
are valuable only where 
read in connection with a 
pair of maximum-minimum 
thermometers in the air, or 
when read in a series of in- 
struments placed above dif- 
ferent soils or plants. 
98. Thermographs. Two 
types of standard instru- 
ments are in general use = = 
for obtaining continuous Fig. 19. Draper thermograph. 
records of air temperatures. 
These are the Draper thermograph, made by the Draper Manufacturing 
Company, 152 Front St., New York city ($25 and $30), and the Richard 
thermograph sold by Julien P. Friez, Baltimore ($50). After careful 
trial had demonstrated that they were equally accurate, the matter of cost 
was considered decisive, and the Draper thermograph has been used ex- 
clusively in the writer’s own work. This instrument closely resembles the 
psychrograph manufactured by the same company. It is made in two sizes, 
of which the larger one is the more satisfactory on account of the greater 
distance between the lines of the recording disk. The thermometric part 
consists of two bimetallic strips, the contraction and expansion of which 
