Operation of the Porous Cup Atmometer. 117 



come to possess any meaning at all, and it makes no diflference 

 what units are used so long as the standard is comparable 

 throughout the series and so described that other observers 

 may duplicate it. 



Coating of cups. — ^It has been found highly advantageous 

 to coat the basal portion of the cups with some water-proof 

 material, partly to avoid clogging of the pores through handling 

 (which should always be done by the coated portion) and 

 partlv to diuiinish the effective evaporating surface and hence 

 the water loss. For humid regions, merely the enlarged rim is 

 coated; for regions of great evaporation, about one-half of the 

 cup is coated. More than three-fourths of the cup should not 

 be coated in any case, since a very rapid clogging of the active 

 surface follows such treatment, at least at the Desert Laboratory. 

 This seems to be due to the accumulation in the active portion of 

 soluble n:aterial from the entire cup. New cups coated to within 

 three centim.eters of the tip have often proved impossible of 

 standardization, the coefficient on successive days rising rapidly 

 from about 1.50 to numbers as great as 12 or 15. 



Of course the end desired is that the loss for a period (as a 

 dav or a week) be large enough so that errors in reading, etc., 

 may be minimized and, at the same time, that an excessive 

 amount of distilled water be not required in the operation. 

 At the Desert Laboratory in June a standard cup loses from 80 

 to 100 cubic centimeters per day. 



A great deal of difficulty was met with in finding a suitable 

 substance for forming the coating. Paraffin scaled off, as did 

 also shellac. Shellac is somewhat soluble in water; various 

 varnishes, enamel paints, and Kotinsky cement are markedly 

 soluble, as is also white sealing wax. Cups coated with these 

 soluble miaterials show sooner or later an area of resinous matter 

 at the tip of the cup, this area gradually increasing, with a cor- 

 responding rise in the coefficient of correction. One of the best 

 substances found was red sealing wax. This is but slightly 

 soluble, and cups treated with it showed no migration of the 

 material to the tip even after months of use. There was no 

 appreciable clogging in the case of red sealing wax. Cups 

 coated with red wax and then painted with white enamel paint 

 gave very satisfactory results. They possessed the desirable white 



