220 TENTH ANNUAL REPORT OP 



PSYCHROMETER. 



Placing. — The psyclirometer, 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 winds, 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 fine- 

 ness, not too coarse; it should form a covering of equal thickness on 

 all sides, and should not be drawn too closaly 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.] 



Ohservation. — 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 tlie 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 incrust the cloth after evaporation. 



[In some arrangements of the psychroraeter the wet-bulb is kept 

 constantly wet by conducting water to it from a small vessel_, by 

 capillary attraction, along a string of cotton wick. A series of com- 

 parative observations were 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 psychro- 

 meter for a moment. 



While the wet bulb is slowly acquiring the temperature of evapo- 

 ration, the observer is occupied with other observations, watching the 

 psychrometer to make sure of the moment when it has become station- 

 ary. In summer, from four to ten minutes are needed for this, accord- 

 ing to the size of the bulb ; but in winter, when the water freezes on 

 the bulb^ it must be moistened from fifteen to thirty minutes before 

 the observation, which should not be made until the ice around the 

 bulb is quite formed and dry. The best way is to keep round the 

 bulb a layer of ice, constant and uniform, which should be neither too 

 thick nor too thin ; then the observation may take place immediately. 

 When the temperature is in the neighborhood of the freezing-point, 

 the observation of the psychrometer requires very peculiar care ; the 

 reason of which we have elsewhere explained. During a fog the wet- 

 bulb thermometer may be somewhat higher than the dry-bulb ; then 



