THE SMITHSONIAN INSTITUTION. 



221 



the air is over-saturated, and contains, besides the vapor at its maxi- 

 mum of tension, water suspended in a disseminated liquid state. 



If the air is very still, it is well to increase the evaporation b}' set- 

 ting the air in motion by a fan. If the wind is too strong, the instru- 

 ment 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 temper- 

 ature and the hygrometrical state of the air issuing from the chamber, 

 which is always ditferent 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 compared 

 from time to time, and if a difference is found, the instruments must 

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

 corrected when entered in the journal. 



BAROMETER. 



■w 



h 



W [ 



v^ 



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 instru- 

 ment 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 the sun, or to the currents of the air, which always 

 take place at the joinings of the windows. When the 

 barometer has to be fixed to the wall, as is the case 

 with all the common stationary and wheel barometers, 

 care must be taken to secure the tube in a position 

 perfectly vertical, regulating it by the plumb-line, 

 first in front, then at the sides, at least in two verti- 

 cal planes cutting each other at right angles. When 

 the instrument is so constructed as to take its equilib- 

 rium itself, as the Fortin barometers and those of J. 

 Green, recently made under the direction of the 

 Smithsonian Institution, it is enough to hang it on a 

 strong hook. These conditions being fulfilled, the 

 rest of the arrangement may be varied according to 

 the nature of the localities. For the Fortin and 

 Green barometers, the following seems to be the most 

 convenient, and may be almost everywhere adopted. 

 (See Fig. 4.) 



A small oblong box, (a h,) some inches longer than 

 the barometer, and a little broader than its cistern, is 

 firmly set against the wall, {w ti/,) near the window, 

 in such a manner as to open in a direction parallel to 

 the panes ; at the summit (a) it has a strong hook, 

 (/i h',) which extends beyond the box about two or 

 three inches, and on which the barometer is suspended. 

 The instrument remains generally in the box, which 



