366 ENVIRONMENTAL CONDITIONS. 



The bolometer and the pyrheliometer seem not yet to have been em- 

 ployed in climatological comparisons and the initial cost of these 

 instruments, as well as the labor required in obtaining readings there- 

 from, make it probable that they will not, in their present forms at 

 least, be of much climatological value. It must be remembered that, 

 for such studies as we are considering, the operation of the instruments 

 must be simple, the results must be satisfactory, and the cost must be 

 low. 



Livingston^ has discussed and compared various other simple forms 

 of instruments intended for measuring light intensity, including the 

 Hicks solar radio-integrator, two forms of actinometer employing 

 photographic paper, and his own radio-atmometer. Reference should 

 be made to his paper for what little has been done by way of comparing 

 the readings obtained from these instruments with the corresponding 

 degrees of light influence upon plant transpiration. This extremely 

 important subject is deserving of much further study. 



The Hicks instrument is faulty in its theoretical conception, in 

 several ways; its readings are as much determined by air-temperature 

 as by the intensity of the light which it aims to measure, and they are 

 also greartly influenced by the changing amounts of Kquid in the 

 exposed bulb, in the shaded reservoir, etc. 



The various forms of photographic-paper actinometers, such as the 

 Wynne photographic exposure meter, the Clements ''photometer,"^ 

 and the instrument employed by Wiesner^ are all valuable in measur- 

 ing and comparing sunlight intensities with reference to their possible 

 photochemical effect upon the particular paper or film employed, but 

 they show nothing in regard to the corresponding possible photo- 

 synthetic or evaporational influence upon plants, since the photosyn- 

 thetic process in plants has nothing essentially in common with the 

 photochemical alteration of silver salts, excepting that both are 

 photochemical, and transpiration has nothing at all in common with 

 the photographic process. 



Livingston's radio-atmometer has been greatly improved since 

 its first description,^ and it seems probable that this instrument may 

 prove to be of very great value in climatology as well as in plant 

 physiology, ecology, agriculture, etc., but its general employment in 

 such lines of study is yet to be accomplished. 



The most improved form of the radio-atmometer consists of two 

 spherical porous-cup atmometers, one of the spheres being black and 

 the other white, but the two being otherwise alike . These are separately 

 mounted and are operated side by side in the location where the light 



^ Livingston, (1911a). See also, for the best sunshine records yet available, Brigga and 

 Shantz, 1916. 



^ Clements, F. E., Research Methods in Ecology, Lincoln, Nebraska, 1905. 

 ^ Wiesner, J., Der Lichtgenuss der Pflanzen, Leipzig, 1907. 

 * Livingston, 1915, b. 



