463 



reservoir of mercury is covered with a light casing of the finest cambric, 

 sewed on in a single roll. The cambric is kept continually wet by a 

 small glass fountain, which preserves a constant level of distilled water, 

 and discharges it on the cambric, very little faster than it evaporates, 

 by means of a wooll en thread reaching from the fountain to the cambric ; 

 any redundant drops are received in a little glass basin below. Thus 

 we have a Psychrometer always ready for observation, the wet bulb 

 showing the depression, and the dry bulb, in its silver cylinder, affording 

 the other element required for calculating the fraction of saturation, 

 "When the instrument is to be thus employed, the half silver cylinder, 

 not being required, is to be removed to one side. Both silver surfaces 

 should be kept as highly polished as possible ; the wet point of a finger, 

 with a very small portion of rouge (peroxide of iron), gently rubbed on 

 the surface until dry, will leave it brilliant. When not in use, both should 

 be kept continually covered with chamois-leather cases, any condensed 

 moisture having been previously wiped off.* 



The parts of the instrument are so placed with regard to each other, 

 that, being comprised within a small compass, the whole may be covered 

 by a French shade as a protection against dust and corrosion; but 

 the French shade contributes to purposes of greater importance. M. 

 Hegnault observes — " "When observations are made in the open air, it 

 becomes evident how much the hygrometric state varies from one instant 

 to another, in consequence of incessant changes of temperature. When 

 the Hygrometer is maintained at the dew-point, the silver is observed to 

 tarnish, and resume its lustre, according as the lightest breath comes 

 from one side or the other." I may add that, on this account, it is often 

 difficult or impracticable to discover what the dew-point is which truly 

 represents the condition of the atmosphere. I have repeatedly encoun- 

 tered this difficulty. On one occasion it had been raining incessantly 

 for seven hours ; on working the exhausting syringe for a short time, 

 the silver would suddenly become white all over, and in a few moments 

 after it would become perfectly bright, although the mercury had been 

 kept stationary all the time. It is on account of the continual changes 

 which take place in the atmosphere that the indications of the Hygro- 

 meter are so uncertain and difficult to be ascertained in the open air — 

 the proper place for observation. The passage of clouds, or even their 

 vapours over the sun, will cause a perceptible rise or fall of temperature, 

 according to the density of the cloudy and the changes will be easily 

 discoverable by the varying effect of the syringe on the silver cylinder. 



The French shade, if used in the following manner, will obviate 

 the effects of these currents and damp breezes. The French shade be- 

 ing removed, let the syringe be cautiously used until the silver cylinder 

 become dull. After a few moments, its lustre will be recovered, even 

 though its Thermometer be kept stationary by a few short strokes of 

 the piston ; for the mercury is parting with the residue of the heat 



* Figs. 1, 2 in Plate XXX. give a correct idea of the whole instrument. 

 B.I. A. PEOC. — VOL. X. 3 Q 



