PSYCHROMETER. 



PSTCHROMETER. 



Placing. — The psychrometer, or wet-bulb thermometer, must 

 be situated under the same conditions as the thermometer. It 

 should be placed on the same wooden bars, several inches off, 

 and outside of the thermometer. {See Fig. 1.) 



The bulbs should also be entirely free, and at a distance from 

 the bars. 



In case of violent Avinds, the instrument may be sheltered by 

 the movable blind, which may also serve as a fan to promote 

 evaporation when the air is too still. 



The cloth which surrounds the bulb ought to be of medium 

 fineness, not too coarse ; it should form a covering of equal 

 thickness on all sides, and should not be drawn too closely upon 

 the glass. Linen is preferable to cotton, which retains the dust. 

 The covering should be changed every two or three months, and 

 the bulb cleaned. [The linen may be washed, without removal, 

 by means of a jet of clean water from a small syringe.] 



Observation. — For the observation, take first a small vessel 

 full of water, which should be left on the window, that the water 

 may be at the temperature of the air ; bring it near to the bulb, 

 and immerse the bulb several times into the water. All the space 

 between the bulb and the bottom of the scale must be wet, and 

 care must be taken that the wrapping is thoroughly moistened, 

 without, however, a too large drop remaining suspended at the 

 bulb. The water used must be pure ; the best is rain-water, 

 filtered, because it does not hold any salt in solution, which 

 might inerust the cloth after evaporation. 



[In some arrangements of the psychrometer, the wet-bulb is 

 kept constantly wet by conducting water to it from a small ves- 

 sel, by capillary attraction, along a string of cotton wick. A 

 series of comparative observations was made at this Institution, 

 last summer, on these two modes of wetting the bulb, which gave 

 the same result within a fraction of a degree from the mean of 

 the records of a month. The observers connected with the 

 Coast Survey prefer the method of dipping the covered bulb.] 



After wetting the bulb, shut the window, and leave the psy- 

 chrometer for a time. 



While the wet bulb is slowly acquiring the temperature of 



