64 BURTON EDWARD LIVINGSTON 



It is clearly possible, to decrease the amount of unimbibed 

 water thus in general contact with the porous plate, per unit of 

 surface, and thus to bring the two groups into closer similarity. 

 The amount of this unimbibed water backing the solid, support- 

 ing material is a matter of considerable importance; for the tem- 

 perature of the active surface, an important feature in deter- 

 mining rate of water loss, must follow temperature changes in the 

 surroundings less promptly {i.e., with greater lag) the greater 

 is the mass of water present behind the evaporating films. It 

 was for this very consideration that the paper cylinder and its 

 mounting were devised; it appeared clear that the porous clay 

 cup, with its relatively thick, imbibed walls and its propor- 

 tionally large amount of free water, should be less sensitive to 

 external changes than such a water-holding device as the paper 

 cylinder. For example, the temperature lag of the paper surface 

 should be induced only by the small water mass actually held 

 in the paper by imbibition or capillarity. In this respect such a 

 surface should more nearly approach the form of exposure offered 

 by ordinary thin leaves of plants. The porous clay cup should 

 act in this regard more as do very succulent leaves, joints of 

 cactus, etc., since here is a relatively large amount of water with 

 relatively smalt surface. 



This matter was made the subject of a special study, in which 

 paper cylinders and porous cups were operated side by side, and 

 simultaneously subjected to sudden and rather great changes in 

 their surroundings, these changes being accomplished by moving 

 the apparatus from one position to another, from sunshine to 

 shade, from the open to a closed room, etc. From these tests it 

 appeared that the lag exhibited by the cup is exceedingly slight, 

 so slight as to be quite negligible unless readings are taken every 

 minute or oftener. On this account it has not yet seemed neces- 

 sary to develop the paper cylinder farther than has been done. 

 Sooner or later this form of evaporating surface may assume 

 importance, as problems become more refined. 



