ATMOMETRY AND THE ATMOMETER 59 



reservoir. It should be emphasized, however, that, whereas the 

 paper forms above mentioned depend almost wholly upon lat- 

 eral or longitudinal movement of water through the absorbent 

 material, the Bellani plate transmits water perpendicularly to 

 its surfaces, from one side to the other. 



The Bellani surface is exposed like free water, horizontally, 

 or may have any other position. It needs no projecting rim 

 and of course waves and splashing cannot occur. At the same 

 time, the relation of the exposed surface to the water mass be- 

 low is not very different (especially as regards heat conditions) 

 from the similar relation for an open pan. Furthermore, as 

 has been seen, this arrangement allows very small readings to be 

 made, by volume units, as for short periods of time or periods of 

 low evaporation intensity. 



This Bellani form of surface is worthy of the attention of those 

 desiring a horizontal, plane evaporating surface, not altered by 

 wind. It may be rendered non-absorbing, and thus freed from 

 rain influence, by the use of the non-absorbing mounting to be 

 described below. As here illustrated, it is conveniently mounted 

 to take water from a burette reservoir. To the kind of altera- 

 tions which may occur in saturated solid surfaces in general, 

 and to ways of combatting such alterations, we shall turn pres- 

 ently. 



The Porous Clay Cap Atmometer (Figs. 5-8). 



This instrument, 11 which has formed the basis for the ex- 

 tremely rapid development of atmometry among biologists, 

 during the past eight or nine years, was independently devised 

 three times before its possibilities became generally appreciated. 

 Babinet's original short account, and Marie-Davy's 12 apprecia- 

 tive discussion, the latter appearing twenty years later than 

 Babinet's, apparently attracted no serious attention. Forty- 

 six years after Babinet's publication Mitscherlich 13 independently 



11 Babinet, J., Note sur un atmidscope. Compt. Rend. Paris 27: 529-530, 1848. 



12 Marie-Davy, H.,'Atmidometre a vase poreux de Babinet. Nouv. Met. 2: 

 253-254, 1869. 



13 Mitscherlich, A., Ein Verdunstungsmesser. Landw. Versuchsst. 60:63-72, 

 1904. 



