464 



•which it had not given off during the first action of the syringe. Let 

 the French shade be now put on, and let the syringe be cautiously 

 worked again until the silver be rendered dull ; one or two short strokes 

 may suffice. The included Thermometer will at this moment indicate 

 the dew-point. So speedy is the abstraction of heat permitted to be 

 by the long slender shape of its mercurial reservoir, that the second 

 dulness may generally be relied on as indicative of the final reduction 

 to the temperature of the dew-point. 



When the French shade is thus used, care must be taken that the 

 air contained in it be at the same temperature and hygrometric condi- 

 tion as the air in which the experiment is made. The best way is to keep 

 the French shade standing inverted, for a few minutes previously, near 

 the Hygrometer. 



Other advantages attend these arrangements: — The French shade 

 prevents the commixture of aqueous exhalations from the breath and 

 person of the operator with the air under trial. The operator may be 

 as close as is necessary to the Hygrometer ; he is relieved from the em- 

 barrassment of observing the descent of the mercury, and the formation 

 of the first cloudiness on the silver at the same moment that he is occu- 

 pied in regulating the stopcock of the aspirator, and doing all this at 

 such a distance as to require the aid of a small telescope. In this mode 

 of proceeding the telescope, the aspirator, its support, and the vessel of 

 water are rendered unnecessary. 



Many experiments have convinced me that the occasional and almost 

 momentary adoption of the French shade has no effect in complicating 

 the results ; that it merely obviates the uncertainty arising from sudden 

 gusts of wind, carrying variable quantities of vapour, and prevents the 

 breath and exhalation of the operator from coming into immediate con- 

 tact with the silver indicator, although, by mixing with the ambient 

 air, it must ultimately do so ; bat not until it has been brought to the 

 temperature of the vaporized ether by passing through it. 



The grand difficulty in using all Hygrometers on the condensing 

 principle is that of ascertaining with precision the moment when the 

 first cloud appears on the polished silver or glass ; practice will over- 

 come it. A choice of situation with regard to the direction of the light 

 is important. Although I have applied the word cloud to the change 

 induced on the silver cylinder by condensation of the aqueous vapour 

 on it, there really ought to be no actual obscuration of the polish; the 

 Thermometer would then be below the dew-point. The change should 

 not amount to more than an alteration in the hue of the silver, to be 

 observed best by comparison with the half cylinder. The polish of both 

 should be exquisite. 



For the purpose of meteorological research, the apparatus, as 

 hitherto described, would be difficult to manage, if not impracticable. 

 To meet the exigencies of an open country, a mountain district, or the 

 depths of a mine, the arrangements must be different. For these pur- 

 poses, the part of the Hygrometer Avhich contains the enclosed Thermo- 

 meter and silver cylinder must be unscrewed from the stand, the 



