24 KGGS IN COLD STORAGE. 



with rnuslin so as to retain a film of water surround- 

 ing it. The working- of this instrument depends on 

 a law which may be roughly expressed, as "evapora- 

 tion carries off heat." The evaporation of water from 

 the bulb incased in muslin, known as the wet bulb, 

 cools it somewhat, depending- on how dry the air sur- 

 rounding- it may be. The difference between the 

 reading- of the wet bulb thermometer and the reading 

 of the dry bulb thermometer, when compared with 

 reference to a prepared table, give's the relative 

 humidity of the air at the time of making- the observa- 

 tion. Psych rometers are of two kinds, stationary 

 and sling-. 



The stat i nar y psychrometer is essentially like 

 the sling- psychrometer, both depending- on the same 

 principle. The sling- instrument is more compact 

 and provided with a handle for whirling-, while the 

 stationary instrument is intended to be fastened 

 against the wall, or on a post, the muslin covering- the 

 wet bulb being connected by a porous cord with a 

 reservoir of water, to keep the supply of water con- 

 tinuous. This is essential, as it takes some little 

 time to obtain a correct reading with this pattern of 

 instrument. For this reason it is open to the same 

 objections as the hygrometer. Also, after short use 

 the muslin covering the wet bulb, and the cord feed- 

 ing water to it, become clogged with solid matter and 

 fungous growth affecting its accuracy. At any tem- 

 perature below 32 F. this instrument is useless, as 

 the water will freeze in the cord supplying the mus- 

 lin on the wet bulb, and the muslin becomes dry in 

 consequence. 



For practical, accurate and quick results at any 



psychrometer. . 



temperature there is no instrument so reliable and 

 convenient as the sling psychrometer, preferably of 

 the pattern known as Prof. Marvin's improved psy- 

 chrometer, shown in the illustration. This is a 

 standard Weather Bureau instrument, and when used 



