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. Tliermographs. Two 



types of standard instru- 

 ments are in general use 

 for obtaining continuous 

 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 



Fig. 19. Draper therniiigr.iijU. 



