296 BURTON E. LIVINGSTON AND EDITH B. SHREVE 



the ordinary atmometer cup/ This particular kind of cup serves 

 very well here, but needs to be ground off at the closed end so 

 as to form a flat surface somewhat over 1 cm. in diameter and 

 perpendicular to the long axis of the cup. If the grinding be 

 too long continued the end wall of the cup will of course be com- 

 pletely removed, but this wall is sufficiently thick to allow the 

 formation of the small plane surface here required. 



An opening about 8 mm. in diameter is drilled in the side 

 of the cup, about 1 cm. from the closed end, and a thermometer 

 bulb inserted, with a small rubber stopper. It is well if this 

 thermometer has a bent stem, the scale portion being vertical 

 and about 7 cm. distant from the horizontal bulb, which is 

 within the cup. The whole outer surface of the cup excepting 

 the flat top is thoroughly waterproofed with shellac or melted 

 sulphur, to prevent water loss anywhere except through the 

 plane surface at the top. 



The upper portion of the cup is now set into the center of a 

 piece of wood about 10 cm. square and 2 or 3 cm. thick, so that 

 the plane top of the wood block is continued at the center by 

 the flat porous clay plate formed by the top of the cup (see fig- 

 ure 1). The cup is held in place by sealing-wax or a similar 

 cement. It is best not to finish grinding the porous surface 

 until the cup is in place, and to fix the latter so that the top pro- 

 jects a httle above the wood surface. The projecting portion 

 of porous clay is afterwards removed by grinding, and the en- 

 tire surface (of wood and clay) is made plane. 



The horizontal part of the thermometer is set into a suitable 

 groove in the lower side of the wood block. A rubber tube is 

 placed about this portion of the stem, and then the groove is 

 filled with wax cement, to retain the thermometer in place. The 

 whole wood block is heavily water-proofed with shellac. 



To provide the milUmeter of vapor blanket so far used over 

 the standard evaporating surface, a plane sheet of metal, hard 



^ Livingston, B. E., The relation of desert plants to soil moisture and to evapo- 

 ration. Carnegie Inst. Wash. Publ. 50. Washington. 1906. Atmometry and 

 the porous cup atmometer. Plant World 18: 21-30, 51-74, 95-111, 143-149. 1915. 

 Also reprinted, Tucson, 1915. 



