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. [Illustration: Fig. 17. Maximum-minimum thermometer.] [Illustration: Fig. 18. Terrestrial radiation thermometer.] [Illustration: Fig. 19. Draper thermograph.] =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 directly above the soil or plant to be studied. It is otherwise 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 consequence, 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 instruments placed above different soils or plants.