METHOD FOR DETERMINING TRANSPIRING POWER 297 



rubber or pressed fiber of this thickness, and with a rectangular 

 opening at its center, 4x8 mm., is placed upon the upper sur- 

 face of the wood block, the opening lying over the exposed porous 

 surface. A glass plate overUes this, covering the cobalt paper 

 slip in the manner heretofore used with the Harvey blotting- 

 paper surface."^ The millimeter plate may be attached to the 

 wood if this seems desirable, but it should not be allowed to 

 buckle with rise in temperature. A good way to attach it is 

 by means of four thumb-tacks placed outside of its margin, 

 neai* the corners. 



The porous cup is filled with distilled water and mounted as 

 with the ordinary absorbing mounting of the atmometer.'^ 

 The glass tube is only about 8 cm. long here, however, and the 

 small reservoir bottle is brought as close as possible to the porous 

 cup. Care should be exercised in filling to have no air-bubbles 

 in the cup. The complete apparatus is conveniently supported 

 on a small tripod ring, the bottle hanging below the wood block, 

 which rests upon the ring. 



The apparatus just described furnishes a suitable standard 

 evaporating surface for laboratory and greenhouse use, but is 

 not convenient for the field. The temperature to be considered 

 in the operation of this instrument is the temperature of the 

 evaporating surface itself and the placing of a glass and mercury- 

 thermometer so as to give readings of this temperature is diffi- 

 cult or impossible. The temperature determined by means of 

 the thermometer in the apparatus just described is not precisely 

 that of the evaporating surface; it is simply the temperature of 

 the mass of water that bathes the porous clay plate from with- 

 in the cup. It is thus seen that the instrument here recom- 

 mended for laboratory and greenhouse is not quite perfect in 

 this particular; there is room for improvement. 



For field use, it is inconvenient to employ any arrangement 

 of the standard evaporating surface that has a thermometer 

 as an essential part. During the years when the use of stand- 

 ardized cobalt-chloride paper has been developing, many differ- 



* See Livingston, 1913, I.e. 

 '' See Livingston, 1915, I.e. 



