ATMOMETRY AND THE ATMOMETKK 10.") 



absorbing regions. Should this occur, it is clear that the nature 

 of the evaporating surface would be profoundly altered, through 

 changed water content of the porous porcelain at the affected 

 place. Furthermore, if this drying-out of the cup wall proceeds 

 far enough it may finally remove some portion of the water 

 menisci which prevent the entrance of gaseous air, and the cup 

 and tube must soon become emptied of water, all the unim- 

 bibed liquid escaping to the reservoir. 



Whether any error in reading may arise from air in the cup 

 depends upon the amount of air thus present and upon the at- 

 mospheric evaporating power at the time. With low evapora- 

 tion rates longitudinal water movement through the affected 

 part of the wall may suffice to keep it normally saturated. With 

 a given rate of water loss there must be a limit of size to which, 

 but not beyond which, the air mass in the cup may be increased. 

 In the practical operation of the instrument, undissolved air 

 should not be allowed to enter the cup. While small gas bubbles 

 are undoubtedly harmless, yet it is the only safe procedure always 

 to keep these as small as possible or entirely absent. 



Operation of the Porous Cup Atmometer 



Cleanliness. The most important point to be remembered in 

 the operation of porous cup atmometers is that they should be 

 kept as clean and free from extraneous material as possible. 

 This proposition leads, first to the use of nothing but good 

 distilled water. With impure water trouble is sure to occur. In 

 handling the cups they should never come in contact with the 

 hands excepting on the shellacked or glazed portion. 



If atmometers are to be operated near to the soil surface, the 

 danger of injury from dust and from mud thrown upon the cups 

 by rain is greatly increased. If the soil about the instrument 

 is covered, as by stones or vegetation, this danger is largely 

 avoided To remove dust, insects, etc., frequent washing of the 

 cup is advisable, rubbing it gently with a soft sponge and plenty 

 of distilled water. In the case of shellacked cups the rubbing 

 should be downward, to avoid transferring the slightly soluble 

 shellac to the porous surface. With the glazed cups this cleaning 



