ATMOMETRY AND THE ATMOMETER 149 



such adjustment might be avoided through the use of spherical 

 porous cups. Such cups, with a cylindrical neck projecting 

 downward, for attachment to the water tubes, should present 

 the same range of angle of incidence to the sun's rays at all 

 times of the day and year; at all times during sunlight a hemi- 

 sphere of surface must be illuminated. The obtaining of black 

 and white spheres, of correct size, porosity, etc., was then, of 

 prime importance for the development of radio-atmometry. For 

 several years the writer has been endeavoring to have the requi- 

 site spheres manufactured and perfect ones have not yet been 

 obtained. Quite independently, Prof. W. L. Tower, 29 of the 

 University of Chicago, has obtained black and white spheres, 

 which he has had in use for considerable time, with ver}^ satisfac- 

 tory results. It is hoped that spherical porous cups may soon 

 be placed upon the market. When this becomes possible, the 

 development of the radio-atmometer should proceed rapidly. 



With the advent of spherical cups a new standard will need 

 to be adopted, for of course it will not be possible to establish 

 a generally constant relation between the sphere and the cylin- 

 drical cup or between them and any water surface. As far as 

 ordinary atmometry is involved, however, there appears no 

 reason why the cylindrical cup may not be as satisfactory as 

 the white sphere. It is only when radiation intensity is to be 

 studied that spherical cups have any considerable advantage 

 over the ordinary cylindrical type. 



29 Announcement of Prof. W. L. Tower's work in: MacDougal, D. T., Annual 

 report of the Director of the Department of Botanical Research. Year Book 

 Carnegie Inst. Wash. 12: 57-87. 1914. Page 71. 



