BAROMETER. 9 



evaporation, the observer is occupied with other observations, 

 though watching the psychrometer to make sure of the moment 

 when it has become stationary. In summer, from four to ten 

 minutes are needed for this, according 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 imme- 

 diately. 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 sometimes be higher 

 than the dry-bulb ; then the air is over-saturated^ and contains, 

 besides the vapor at its maximum of tension, tvater suspended in a 

 disseminated liquid state. This is, however, not a frequent oc- 

 currence. 



If the air is very still, it is well to increase the evaporation by 

 setting the air in motion by a fan. If the wind is too strong, 

 the instrument should be protected by the movable blind. The 

 reading must be made rapidly, and, as much as possible, at a 

 distance, and without opening the window ; for the proximity of 

 the observer, either by the heat radiating from his body, or by 

 his breath, as well as the temperature and the hygrometrical state 

 of the air issuing from the chamber, which is always different from 

 that of the external air, especially in winter, would infallibly act 

 upon the instruments, and would falsify the observation. 



Verification. — The two thermometers must be carefully com- 

 pared from time to time, and if a difference is found, the instru- 

 ments must be adjusted, or it must be taken into the account, and 

 the observations corrected when entered in the journal. 



BAROMETER. 



Placing. — The barometer should be placed in a room, of a 

 temperature as uniform as possible ; not heated, nor too much 

 exposed to the sun. The instrument must be suspended at the 

 height of the eye, near a window, in such a manner as to be 

 lighted perfectly, without exposure either to the direct rays of 



