HYGROMETRY. 



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- i. UNDER the article EVAPORATION, we explained the 

 numner in which wter is supposed to be derated, and 

 suspended in the atmosphere: we now propose to take 

 new of the various contrivances which have been em- 

 ployed tor detecting the presence of aqueous vapour 

 in that fluid, *nd ascertaining, not only the relative, 

 but the absolute quantity of moisture, which exists at 

 different times in given portions of air. 



The foundation of almost all the contrivances 

 which have hitherto been employed for that purpose, 

 is the affinity for moisture possessed by a variety of sub- 

 stances. This affinity is exerted more or less by a con- 

 iderablc number of bodies ; but it is displayed, in the 

 most eminent degree, by sulphuric acid, the fixed al- 

 kalies, several of the earths, the salts denominated deli- 

 quescent, more especially the muriate and nitrate of 

 lime ; as well as by many substances of animal or ve- 

 getable origin, particularly hair, membrane, hum, ivory, 

 whalebone, feather*, the beards of corn, wood, ionl- 

 a/e, paper, Arc. All these substances possess the pro- 

 \#ny <if abstracting nioiture from the atmosphere; 

 i>ut the union which they form with it is so very slight, 

 that they readily yield it up again to the air when tli.it 

 : has by any means become drier, either by an in- 

 crease of temperature, or the deposition of the water 

 which formerly existed in it in a vaporous state. I 

 the condition of these bodies with respect to humidity, 

 may be employed as an indication of the <jiia; 

 ntotHure contained in a given volume of the air by 

 which they are surrounded. The epithet IlygrompK 

 ha* been applied to substances used for tf 

 rxl the various instruments which have been formed 

 of them, are called Hygromeifrt, or meanrtri of moit- 

 turr. 



And hrgfo- S. The alternations of dry-ness and humidity to which 

 all hygroscopic bodies are subject, are accompanied with 

 corresponding changes in their weight and dimension*; 

 and. therefore, all hygrometer* are constructed so a* to 

 have a reference by their indication*, either to change 

 of weight or change of volume The latter of the** 

 change*, though frequently lew appreciable in its ex- 

 tent than variation of weight, is however more readily, 

 a* well as more conveniently estimated ; and hence the 

 greater number of hygrometer* comritt of some animal 

 or vegetable tubitance, having a fibroin structure, the 

 expansion* of which by moisture are rendered more 

 senible by mechanical contrivance*. Some hygro- 

 meters, however, are constructed, *o a* to indicate 

 chalice*) of humidity in the medium to which they 

 are rxpoted, by changes of weight, arising from the 

 absorption or extrication of moisture ; and one of these 

 in*trumem. invented by Mr I^slie, perhap. the most 

 accurate of them all, i formed upon a principle v 

 cannot be referred either to change of weight or of vo- 

 lume. 



4. It i milyof late that hygrometer* have been con- 

 itruc iy degree of accuracy. The earlier in- 



which bear that uamc, were extremely im- 

 perfect : the mechanical part wa executed in a very 

 rode tninni-r, and no attention wa* paid to the gradu- 

 ation of the *cale which marked the enlargement or 

 contraction of the hygroscopic lubttance, farther than 

 to make it point out mere differences in the state of the 

 PART ii. 



PLATE 

 CCCXXI. 

 fig. 1. 



air with rwpect to moisture. No attempt was made to 

 determine two fixed points, as in the case of the ther- 

 mometer, by which the various scales might be reduced 

 to a common standard ; and still less to ascertain the 

 absolute quantity of moisture in a given volume of air, 

 corresponding to the different points of the scale em- 

 ployed. In short, nothing higher was aimed at in the 

 iction of a hygrometer, than to obtain some sub- 

 stance which suffered considerable variations of bulk, 

 by the absorption of moisture, without the smallest re- 

 gard to the regularity of its dilatations. Almost all of 

 them were very unwieldy ; and none of them could be 

 applied to nice researches into the hytfrometric state of 

 small portions of air. To describe the construction of 

 them, with much minuteness, would therefore be as 

 useless as inconsistent with the limits of this article. 



5. Both animal and vegetable substances of n fibrous Conirr'j 

 structure, possess the property of being dilated by mois- hygrometer, 

 ture, in a direction transverse to the fibres ; and, ac- 

 cordingly, the lateral expansion of these bodies fur- 

 tlie principle upon which a considerable number 

 Tometers are constructed. One of the earliest 

 i-trr- >!' thii kind was proposed by Mr Coniers 

 1 he hole contrivance is of the rudest kind ; 

 ami though it is scarcely worthy of notice, we shall 

 give a brief description of it, in order that some idi-a 

 may be formed of the imperfect state of hygrometry at 

 that period. A A A A represent* a frame of wood grooved 

 in the inid- lor admitting two pannels of deal 15, H to 

 play freely at top an The pannels. which are 



placed so as to have their fibres in a vortical position, 

 are fastened to the frame at each side, and a sufficient 

 interval is left between them, to allow full scope for 

 the wood to dilate itself in a lateral direction. The axis 

 of the index, which is at C, by receding from F, or 

 approaching nearer to it, gives a circular motion to the 

 v itself, by means of a slender metallic chain, which 

 passe* round the axis, and is fastened to one of the 

 panncU at F. W i* a weight or counterpoise connect- 

 ed with the axis by mean* of a string passing over the 

 pulley I), and attached to the arm (.'(>, and which 

 cause* the index to descend, as the pannels expand I y 

 moisture. Several other contrivances of a similar kind 

 are described in thr Philosophical Transactions for that 

 period, but they are all equally rade and imperfect. 

 The most accurate hygrometer, constructed on the 

 .f a lateral expansion, is that ot'De I.uc. The 

 sulwtancr he employs in preference to all others, is 

 whalebone cut tran*vcr*ely into thin *lip. Such is the 

 tenacity of thi* substance, that, according to DC- I.uc, 

 these slip* may be a foot long, and a line in breadth, 

 without weighing above one-fourth of a grain, and yet 

 be capable of supporting a weight of about lb'0 grams. 

 The instrument is fitted up in various forms by diffe- 

 rent artist* ; but the general principles of its construc- 

 tion are nearly the same in all. Mr Adic, Edinburgh, 

 construct* it in the following manner. Alt' - Fig. ? 



ents the frame-work of the instrument, to the upper 

 part of which is attached the graduated < . ca- 



pable of being elevated or lowered at pleasure, to suit 

 tin I. ngth of the slip of whalebone a l>. The whale- 

 bone, which is usually about 10 inches in length, i* fas- 

 tened at a between two slips of brass by means of a 

 c 



Whalebone 



hyirranMter 

 u DC Luc. 



