Fig. 5. Sling psy- 
chrometer. 
THE HABITAT 
decrease in temperature proportional to the amount of 
moisture in the air. The dry bulb thermometer is an 
ordinary thermometer, while the wet bulb is covered with 
a cloth that can be moistened. The former indicates the 
normal temperature of the air, the latter gives the re- 
duced temperature due-to evaporation. The relative 
humidity of the air is ascertained by means of the proper 
tables, from two terms, i. e., the air temperature and 
the amount of reduction shown by the wet bulb. The 
sling and the cog psychrometers alone are in general 
use. The stationary form has been found to be unre- 
liable, because the moisture, as it evaporates from the 
wet bulb, is not removed, and, in consequence, hinders 
evaporation to the proper degree. 
61. The sling psychrometer. The standard form of 
this is shown in the illustration, and is the one used by 
the Weather Bureau. This instrument can be obtained 
from H. J. Green, 1191 Bedford Ave., Brooklyn, or 
Julien P. Friez, 107 E. German St., Baltimore, at a cost 
of $5. It consists of a metal frame to which are firmly 
‘attached two accurately standardized thermometers, 
reading usually from —-30° to 130°. The frame is attached 
at the uppermost end to a handle in such fashion that 
it swings freely. The wet bulb thermometer is placed 
lower, chiefly to aid in wetting the cloth more readily. 
The cloth for the wet bulb should be always of the same 
texture and quality; the standard used by the Weather 
Bureau can be obtained from the instrument makers. A 
slight difference in texture makes no appreciable error, 
but the results obtained with different instruments and 
by different observers will be more trustworthy and 
comparable if the same cloth be used in all cases. The 
jacket for the wet bulb may be sewed in the form of a 
close-fitting bag, which soon shrinks and clings tightly. 
It may be made in the field by wrapping: the cloth 
so that the edges just overlap, and tying it tightly above 
and below the bulb. In either case, a single layer. of 
cloth alone must be used. The cloth becomes soiled or 
thin after a few months’ congtant use and should be 
replaced. It is a wise precaution to carry a small piece 
of psychrometer cloth in the field outfit. 
