84 J. W. SHIVE AND B. E. LIVINGSTON 



it will be necessary only that the evaporating power of the air 

 do not surpass it, in order that the soil moisture content at the 

 time of permanent wilting of the plants may be determined by 

 soil conditions alone. This conclusion is based on the results 

 recorded by Briggs and Shantz, to the effect that the residuum 

 of moisture remaining in the soil at permanent wilting was un- 

 influenced by such variations of the aerial conditions as were em- 

 ployed by these authors; they obtained the same residual soil 

 moisture content from wiltings in a glass damp chamber as 

 resulted from wiltings in their unshaded greenhouse, and it is 

 thus apparent that the limit of the evaporating power of the 

 air here postulated must lie somewhat above the evaporating 

 power that prevailed in the unshaded greenhouse of these authors. 

 Caldwell's moist chamber produced first wiltings with soil mois- 

 ture residues not very unlike the calculated residues derived 

 from the Briggs and Shantz formula. A study of Caldwell's 

 published data shows that the observed moisture residues from 

 these wiltings were usually somewhat above the calculated values ; 

 they were seldom identical with the latter and in only one case 

 recorded {loc. cit., page 33, table VIII) was an observed residue 

 less than the calculated, the ratio being 0.98 in this single instance. 

 It thus appears that Caldwell's moist chamber usually gave an 

 evaporating power of the air slightly too high to bring the ob- 

 served and calculated residues into perfect agreement. This 

 evaporating power produced a rate of water loss from the standard 

 porous cup atmometer of from 0.2 to 0.3 cc. per hour, during 

 the hours when wilting occurred (Caldwell, loc. cit., page 33, 

 table VIII). It appears highly improbable that the correspond- 

 ing hourly evaporation rates in the unshaded greenhouse of 

 Briggs and Shantz may have been lower than, or even as low as, 

 0.3 cc. What these latter rates may really have been cannot now 

 be determined, but if they be assumed as greater than 0.3cc. 

 per hour, there then appears a serious problem in the relation 

 between the fesults of Briggs and Shantz on the one hand and 

 those of Caldwell on the other; for the earlier series of experi- 

 ments surely gave excellent agreement between the observed 

 and calculated moisture residues, while the later series indicated 



