HUMIDITY 



41 



be of sufficient accuracy for use in ecological study. The difficulty is that 

 the hygroscopic reaction is inconstant, rather than that the instruments are 

 not sufficiently sensitive. A number of hygrometers have been tested, and 

 in all the error has been found to be great, varying usually from 10-20 per 

 cent. In the middle of the scale they sometimes read more accurately, but 

 tovv^ard either extreme they are very inexact. It seems probable that an 

 accurate hygrometer can be constructed only after the model of the Draper 

 psychrograph. Its weight and bulk would make it an impossible instrument 

 for field trips, and the ex- 

 pense of one would provide 

 a dozen psychrometers. 

 In consequence, it does not 

 seem too sweeping to say 

 that no hygrometer can 

 furnish trustworthy results. 

 Of simple instruments for 

 humidity, the psychro- 

 meter alone can be trusted 

 to give reliable readings. 

 Crova's hygrometer, used 

 by Hesselmann, is not a 

 hygrometer in the sense in- 

 dicated. As it is much less 

 convenient to handle and 

 to operate than the cog 

 psych rometer, it is not 

 necessary to describe it. 



Psychro graphs 



66. The Draper psy- 

 chrograph. A year's trial 

 of the Draper psychro- 

 graph in field and plant- 

 house has left little ques- 

 tion of its accuracy and its great usefulness. Essentially, it consists of a 

 band of fine catgut strings, which are sensitive to changes in the moisture- 

 content of the air. The variations in the length of the band are com- 

 municated to a long pointer carrymg an inking pen. The latter traces the 

 record in per cent of relative humidity on a graduated paper disk, which is 

 practically the face of an eight-day clock. The whole is enclosed in a metal 

 case with a glass front. A glance at the illustration will show the general 



Fig. 7, Draper psychrograph. 



