206 Love avd Smeal : 



noticed that Regnault's work was done under conditions which 

 are now recognised to be extremely unfavourable to the right 

 use of the psychrometer. Much of it was carried out in closed 

 spaces Avhere no air-motion was possible, and the effects of 

 radiation from surrounding walls would be a maximum ; two 

 of the series discussed by Ekholm, the seventh and the fourth, 

 were taken under these conditions. In other cases the exposure 

 TO winds was very different in dift^erent directions, and in one 

 case, the ninth series also discussed, which was taken on a 

 high plateau in the Pyrenees, there was practically no shelter 

 from wind at all. Modern practice proceeds on the understand- 

 ing that moving air is almost essential, but that efficient shelter 

 is to be provided from the direct action of the wind, hence the 

 use of screens such as the Stevenson pattern. In some forms 

 artificial wind is provided. Thus Svensson's work was done 

 with an Assmann ventilated psychrometer,^ a steady flow of air 

 being maintained by a centrifugal pump through a cylinder 

 containing the two tlieriuometers. Different workers appear 

 to hold very diverse opinions as to the reliability of the Ass- 

 mann method.'^ As a control method Svensson used the Sonden 

 hygrometer. '^ which measures the increase of volume produced 

 on saturating the air. On this account, and also because of 

 the different results of his two papers, Hazen severely criticises 

 the whole investigation. We have not ourselves had an oppor-- 

 tunity of using the Sonden instrument. 



It is highly probable, therefore, that the apparent diminution 

 ofy in the observations discussed by Ekholm is due to a failure 

 to comply with the conditions requisite for accurate use of the 

 psychrometer, and this supposition is strongly borne out by the 

 results of our observations, taken under more suitable condi- 

 tions. Such a failure may occur occasionally under any cir- 

 cumstances, but, being of the nature of an accidental rather 

 than a systematic error, it will be properly included among 

 the casual variations of the single constant A. If, on the other 

 hand, the diminution be due to hygroscopic action, it will be 



1. Assmann, Meteor. Zeitschr., 18S>1, p. 15. 



2. Hazen, " Psychrometer Studieii," Meteor. Zeitschr., 1806, p. 27r>. " Das Problem des 

 P.sychrometers," 1809, p. 261. 



3. Sonden, " Ein Nevies Hyj^rometer," Meteor. Zeitschr., 1802, p. Si. 



