A Radio-Atmometer. 97 



increased by the energy absorbed. The white porous clay cup 

 absorbs comparatively little radiation and is thus not af- 

 fected as profoundly as are the plants and the soil. It seemed 

 desirable, therefore, that an intsrument be devised that should 

 absorb as much as possible of the radiant energy of sunshine 

 and should integrate the effects of this absorbed energy in a man- 

 ner as satisfactory as that in which the porous cup atmometer 

 sums the effects of the evaporating power of the air. An attempt 

 in this direction, lasting over several years, has met with some 

 degree of success, and it is to lay the resulting instrument before 

 physiologists, agriculturists, and ecologists that this paper 

 appears. '•' 



The radio-atmometer is merely the common form of porous 

 clay atmometer with the clay surface so modified that it absorbs 

 a considerable amount of the radiant energy falling upon it. 

 Two forms of cup are available, one made of colored clay, the 

 other coated with lamp-black. The former possesses a dark 

 brown color and has the advantage of retaining its color in spite 

 of rain, the latter is dead black. These cups are set up in the 

 ordinary manner, excepting that the position of the cups with 

 reference to the direction of the incident light and heat 

 must be such that the angle of incidence will not vary throughout 

 the period of observation. If this angle be allowed to change 

 there will be introduced errors in the form of apparent varia- 

 tions in the light intensity and the correction for such errors 

 would be laborious and uncertain. The difficulty is readily 

 avoided if we simply so place the cylindrical cup that the sun- 

 shine falls upon it perpendicularly to its long axis at all times 

 of the day, the source of radiation thus virtually rotating about 

 the cylinder in a plane perpendicular to its long axis. The 

 position of the cup thus depends upon the latitude and season. 

 It may pe placed empirically at noon by seeing that it receives 

 the sun's rays perpendicularly, and at the same time that its 

 axis lies in the same vertical plane as the sun. For the northern 

 hemisphere its tip will point more or less to the northward of the 

 zenith. It will need to be readjusted every few da vs. Of 



♦This instrument and its readings are compared to the plant and its rates of water loss, 

 under varied conditions of solar radiation, in an article about to appear in the Botanical 

 Gazette. Other instruments also are there considered. 



