ATMOMETRY AND THE ATMOMETER 63 



of moist soil. Pans filled with moist soil, firmed to a smooth 

 surface, have long been used by students of agricultural water 

 relations. Regarding these, however, we need here make but the 

 following observations. Some difficulty is usually experienced in 

 preventing the soil surface becoming drier as evaporation pro- 

 ceeds, thus producing a profound change in the internal properties 

 of the instrument. This trouble and some others are avoided 

 by the employment of the Bellani arrangement of a horizontal, 

 porous porcelain plate in contact with water below; porous por- 

 celain is in many respects merely a layer of soil, so bound to- 

 gether that it will retain its shape. Thus, the general features 

 of evaporation from such a plate should be in rather close 

 agreement with those of water loss from a smoothly firmed soil 

 surface. 



Porous surfaces, whether of paper, porcelain or any other 

 fixed material, all agree in presenting to the action of the air a 

 water surface mechanically held in position (by surface ten- 

 sion, adhesion and the mechanical rigidity of the material em- 

 ployed) and hence, as has been mentioned, they are not thrown 

 into waves by the wind, so that the danger of splashing which 

 is always imminent with open vessels is completely obviated. In- 

 struments employing such surfaces can be read in terms of either 

 volume or weight and they can be readily arranged so as to give 

 small readings and to expose the evaporating member at some 

 distance from the reservoir. These are the main advantages of 

 such forms of atmometers over those with a free water surface. 



Two Classes of Porous Surfaces 



As has been seen we may consider two different arrangements 

 of porous surface instruments. In one arrangement the satu- 

 rated solid is in contact with unimbibed water for only a small 

 portion of its extent; such forms are the Piche-Cantoni paper 

 disk, the stretched paper or cloth of Pickering and the paper 

 cylinder of the present writer. In the other arrangement the 

 porous solid is backed by a mass of unimbibed water; of this 

 form are the Bellani porcelain plate and the Babinet cup. 



Lj! library]* 



