r 



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. c., 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' constant use and should be 

 replaced. It is a wise precaution to carry a small piece 

 of psychrometer cloth in the field outfit. 



