^^"^^^^i^ 



A SIMPLE NON-ABSORBING ATMOMETER MOUNTING 



EAR.L S. JOHNSTON 

 Maryland Agricultural Experiment Station, College Park, Md. 



Two non-absorbing atmometer mountings have been described, 

 the object of which is to prevent the absorption of rain by porous 

 cup atmometers. Each of these forms has two mercury valves, 

 situated outside the reservoir bottle in Livingston's^ original 

 form, and within the bottle in Shive's^ modification. Both of 

 these mountings require two tubes leading into the cup, one just 

 entering, the other extending to the top of the interior. Suction 

 applied to the outer end of the latter tube tends to create a partial 

 vacuum in the cup and water rises through the other tube, 

 finally filling the cup. This method is unsatisfactory for filling 

 cups with such large pores that air is drawn through the pores 

 before water is raised to the cup from the reservoir. This often 

 occurs even when the cup is wet, if the porous material is so open 

 that the water menisci break before the cup becomes filled. This 

 consideration applies in the case of the very porous black cups 

 recently obtained. 



For use with very porous atmometer cups, the writer has 

 employed a simple modification of the Shive mounting. Its 

 construction does not require the service of a skilled glass-blower. 

 The essential parts are shown in the accompanying figure. 



A piece of glass tubing of about 6 mm. bore and 18 cm. in length 

 (AA', fig. 1) and two other pieces (BB' and CC) that will fit 

 closely into the first are selected. Each of the smaller ones is 



1 Livingston, B. E. A rain-correcting atmometer for ecological instrumentation. 

 Plant World 13: 79-82. 1910. 



^ Shive, J. W., An improved non-absorbing porous cup atmometer. Plant 

 World 18: 7-10. 1915. Livingston, B. E., Atmometry and the porous cup at- 

 mometer. Ibid. 18:21-30, 51-74,95-111,143-149. 1915. ZcZem, Atmospheric 

 influence upon evaporation and its direct measurement. Monthly Weather 

 Rev. 43: 126-131. 1915. 



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