‘HUMIDITY: 4! 
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 
toward 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 
psychrometer, it is not 
necessary to describe it. 
Psychrographs 
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 carrying 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. 
