84 CARNEGIE INSTITUTION OF WASHINGTON. 



Thus the non-absorbing instrument records practically all the evapora- 

 tion occurring during the period of exposure and absorbs practically 

 no water when the evaporating surface is externally covered bj-a water- 

 film, as with rain or dew. The simple mercury valve of Professor 

 Livingston's device has been retained, simplicity and certainty' of 

 operation being the main advantages of this form over any fonn of 

 mechanical valve that might be constructed, but the arrangement of 

 Mr. Shive places both filling and operating valves within the water 

 reservoir, thus producing a self-contained instrument. This makes the 

 non-absorbing form available for all kinds of field studies. The original 

 absorbing form is still employed wherever it is certain that the evapo- 

 rating surface will not be wetted from without. 



Spherical Porous Cups for Atmometry, hy B. E. Livingston. 



Failure of the manufacturers to furnish an adequate supply of 

 Professor Tower's porous spheres for atmometric use^ has led to the 

 repetition of an earlier attempt to procure suitable spheres in the 

 United States. This attempt has resulted in very promising pieces 

 and it is hoped that the required porous spheres, of one make or the 

 other, may soon become available for general employment. The white 

 sphere possesses no advantage over the white cylindrical cup ; a vertical 

 cylindrical evaporating surface seems fully as satisfactory as a spherical 

 surface for measuring the evaporating power of the air. The need for 

 spheres arises from quite another aspect of the general subject of 

 atmometry, namely, from the desirability of measuring the effective 

 intensity of radiant energy, as this influences evaporation from exposed 

 water-imbibed surfaces. The radio-atmometer, devised for this pur- 

 pose, comprises two otherwise similar evaporating surfaces, one of 

 which is white, the other black. The white surface reflects most of 

 the impinging radiant energy, while the black surface absorbs a con- 

 siderable portion. The two surfaces, being similarly exposed, are 

 similarly affected by the evaporating power of the air, and the difference 

 between their evaporational losses is an approximate measure of the 

 radiant energy absorbed by the black one during the period of exposure. 



From the nature of the radio-atmometer it follows that the bundle 

 of intercepted rays, from whatever direction, should always meet the 

 evaporating surface at the same mean angle. Since the sun is the 

 main source of radiant energy with which students of evaporation 

 have to deal, and since the bundle of intercepted rays from this source 

 is continually altering its direction throughout the day and from day 

 to day, with reference to any point on the earth, it is clear that a 

 spherical surface is the only possible one that will intercept the sun's 

 rays always at the same mean angle and not require continuous or 



'See Year Book of Carnegie Institution of Washington No. 12, page 71. 



