572 



HYGROMETRY. 



Hygrome- the degree which it indicates after it has been allowed 



try- to remain a suitable length of time. 



omlUi"'" '** Under hygroscopic instruments constructed on 

 (Mineicr of * ne principle of a general dilatation by moisture, we 

 I'liiuiintllo. may briefly notice the hygrometer of Chiminello, to 

 whom the prize proposed m 1783 by the Academy of 

 Sciences at Manheim for the best comparable hygro- 

 meter was adjudged. The substance he employed was 

 the barrel of a quill, fitted up in the same manner as 

 the ivory hygrometers, already described. The gra- 

 duation of the scale was determined by means of two 

 fixed points. The point of extreme moisture was ob- 

 tained by immersion in water ; that of extreme dry- 

 ness, by exposing the instrument, for the space of four 

 hours, before a moderate fire, at a temperature equal to 

 25 of Reaumur, or 88$ of Fahrenheit. The quill, by 

 exposure to heat, becomes somewhat contracted ; and 

 though the contraction is not so great as would be pro- 

 duced by extreme dryness, Chiminello considered it 

 sufficiently uniform in different quills to serve as a fixed 

 point in his scale. It is obvious, however, that an in- 

 strument graduated in so vague a manner is totally un- 

 fit for any philosophical purpose. 



Reason why jg Having alluded to this instrument, it may be 

 fo'res'eT'ap- wortn y of remark, that it is probably owing to the hy- 

 proaching groscopic property of their feathers that birds are ena- 

 ram, or fair bled to judge of approaching rain or fair weather. For 

 weather. jt is easy to conceive, that an animal having a thousand 

 hygrometers intimately connected with its body, must 

 be liable to be powerfully affected, with regard to the 

 tone of its organs, by very slight changes in the dry- 

 ness or humidity of the air ; particularly when it is 

 considered, that many of the feathers contain a large 

 quantity of blood which must thus be alternately pro- 

 pelled into the system, or withdrawn from it, accord- 

 ing to their contraction or dilatation by dryness or 

 moisture. This view of the subject seems to afford a 

 satisfactory explanation of the extreme sensibility which 

 birds in general shew to coming changes in the wea- 

 ther. 



Haud equidem credo, quia sit divinitus illig 

 Ingenium, aut rerum fatu prudentia major; 

 Verutn, ubi tempestas, ct cceli mobilis humor 

 Mutavere vias ; et Jupiter humidis Austris 

 Densat, erant quiie rara modo, et qua? densa, relaxat; 

 Vertuntur species animorum, et pt'Ctora motus 

 Nunc alios, alios, dum nubila ventus agebat, 

 Concipiunt. 



VmoiL, Geor. lib. i. 415. 



Hat's bind- 17- An hygrometer constructed upon similar princi- 

 tler hygio- pies, but much more delicate in its indications, has 

 Mr latel y been proposed by Mr Wilson of Dublin. The 

 substance he employs is rat's bladder ; which, besides 

 having an extensive range of dilatation, is affected by 

 very slight changes in the hygroscopic state of the air. 

 The scale is graduated by exposing the instrument to 

 air saturated with moisture for the point of extreme 

 humidity; and by afterwards inclosing it in a receiver, 

 over mercury, with a quantity of concentrated sulphuric 

 acid, for the point of extreme dryness. The interval 

 between the range of these two points is then divided 

 into 100 equal parts. These points, however, must ob- 

 viously vary with the temperature of the mercury ; 

 and though this objection, which is applicable to all 

 these mercurial hygrometers, may be obviated to a cer- 

 tain extent by enlarging the diameter of the tube, it 

 OHnnot be entirely removed without diminishing the 

 delicacy of the instrument, unless the precautions, a- 

 dopted by De Luc, be observed in the construction of 



Wilson. 



the scale. The rat's bladder hygrometer is li.ibfc to Wygreu. 

 another objection, which, on account of the ditliculty tr . v - 

 attending the construction of the instrument, is of con- ""V* 

 siderable force: the elevation of the mercury in the 

 tube, by the contraction of the membranous substance, 

 must occasion a pressure in the bladder, which, in some 

 cases, may amount to nine or ten pounds on the square 

 inch, according to the range of the scale. The disten- 

 sion occasioned by such a pressure cannot fail to affect 

 the accuracy of the instrument, and even to expose it 

 to destruction. According to the observations of Lord 

 Gray, this instrument corresponds pretty nearly in its 

 indications with the whale-bone hygrometer of De 

 Luc. 



1 8. An hygrometer depending on the principle of a Allantois 

 general expansion by moisture, and fitted up in the " 

 same manner as the hygrometer of De Luc just alluded ^ 

 to, has been strongly recommended by Jean Baptiste, 

 a Capuchin friar of St Martin at Vicenza. The hygro- 

 scopic substance used in the construction of this instru- 

 ment, is a narrow slip of the allantois of a calf, the thin 

 membrane which envelopes the foetus of animals before 

 birth. The point of extreme moisture was fixed by 

 exposing the instrument to air saturated with aqueous 

 vapour. Another point was determined by heating to 

 the temperature of 50 of Reaumur, or 144^- of Fahren- 

 heit, a small stove, which was kept open, and preserved 

 for some time as nearly as possible at the same tempe- 

 rature, and then introducing the instrument into it, 

 where it was allowed to remain so long as the allantois 

 suffered contraction. According to Jean Baptiste, the 

 degree of dryness obtained by this process is fixed and 

 invariable. The intermediate space on the scale, be- 

 tween the extreme points thus determined, was divided 

 as usual into 100 equal parts. In a subsequent part of 

 this article we shall demonstrate, that no fixed point 

 can be obtained for the graduation of an hygrometer, 

 by a mere exposure of the instrument to an elevated 

 temperature ; and, consequently, the scale of this hy- 

 grometer can no more be relied upon than that of Chi- 

 minello. The substance itself, however, we have reason 

 to think, from the experiments we have made with it, 

 is exceedingly fit for hygrometric purposes. 



19. But of all the hygroscopic instruments which we ^' 

 have hitherto described, and which are constructed on fj^u" ' 

 the principle of a general, or at least of a longitudinal 

 expansion, the hygrometer of Saussure is by far the 



most delicate, as well as the most accurate and uni- 

 form, in its indications. The substance which he se- 

 lected, in preference to every other, was a human hair, 

 the elongations and contractions of which by moisture 

 and dryness, though less extensive than some of the 

 substances already mentioned, may be rendered suffi- 

 ciently sensible by mechanical contrivances. As the 

 value of this instrument has been greatly enhanced by 

 the late researches of Gay Lussac, a particular descrip- 

 tion of it is the more necessary : we shall therefore give 

 an account of the different parts of it in detail. 



20. The general appearance of the instrument is PLATE 

 nearly the same as that of the whalebone hygrometer c( c 

 of De Luc, which seems indeed to have been borrowed s ' 

 from it, the priority of invention being due to Saussure. 



The upright pillars a a a a, which support the dial- 

 plate, are fixed to the rectangular frame bbbb, at each 

 corner of which is a screw for fixing the instrument to 

 the bottom of its case, when it is exposed to the exter- 

 nal air. The dial-plate is made to slide along the pil- 

 lars a a, and is thus capable of being raised to any par- 

 ticular altitude, in order to suit the length of the hair. 





