HYGROMETRY.
the article EVAPORATION, we explained the _ 'panner in which water is supposed to be elevated, and suspended in the atmosphere : we now propose to take a view of the various contrivances which have been em ployed for detecting the presence of aqueous vapour in that fluid, and ascertaining, not only the relative, but the abso lute quantity of moisture, which exists at different times in given portions of air.
2. The foundation of almost all the contrivances which have hitherto been employed for that purpose, is the affini ty for moisture possessed by a variety of substances. This affinity is exerted more or less by a considerable number of bodies ; but it is displayed, in the most eminent degree, by sulphuric acid, the fixed alkalies, several of the earths, the salts denominated deliquescent, more especially the muriate and nitrate of lime ; as well as by many substances of animal or vegetable origin, particularly hair, membrane, horn, ivory, whalebone, feathers, the beards of corn, wood, cordage, paper, &c. All these substances possess the pro perty of abstracting moisture from the atmosphere ; but the union which they form with it is so very slight, that they readily yield it up again to the air when that fluid has by any means become drier, either by an increase of tempera ture, or the deposition of the water which formerly existed in it in a vaporous state. Hence the condition of these bo dies with respect to humidity, may be employed as an indi cation of the quantity of moisture contained in a given vo lume of the air by which they are surrounded. The epithet Hygroscopic has been applied to substances used for this purpose ; and the various instruments which have been formed of them, are called _Hygrometers, or measurers of moisture.
3. The alterations of dryness and humidity to which all hygroscopic bodies are subject, are accompanied with cor responding changes in their weight and dimensions ; and, therefore, all hygrometers are constructed so as to have a reference by their indications, either to change of weight or change of volume. The latter of these ghanges, though
frequently less appreciable in its extent than variation of weight, is however more readily, as well as more conve niently estimated ; and hence the greater number of hygro meters consist of some animal or vegetable substance, hay nig? fibrous structure, the expansions of which by mois ture are rendered more sensible by mechanical contri• vances. Some hygrometers, however, are constructed so as to indicate changes of humidity in the medium to which they are exposed, by changes of weight, arising from the absorption or extrication of moisture ; and one of these in struments, invented by Mr Leslie, perhaps the most accu rate of them all, is formed upon a principle which cannot be referred either to change of weight or of volume.
4. It is only of late that hygrometers have been construct ed with any degree of accuracy. The earlier instruments which bear that name were extremely imperfect : the me chanical part was executed in a very rude manner, and no attention was paid to the graduation of the scale which marked the enlargement or contraction of the hygroscopic substance, farther than to make it point out mere differ ences in the state of the air with respect to moisture. No attempt was made to determine two fixed points, as in the case of the thermometer, by which the various scales might be reduced to a common standard ; and still less to ascer tain 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 con struction of a hygrometer, than to obtain some substance which suffered considerable variations of bulk, by the ab sorption of moisture, without the smallest regard to the re gularity of its dilatations. Almost all of them were very unwieldy; and none of them could be applied to nice re searches into the hygrometric state of small portions of air. To describe the construction of them, with much minute ness, would therefore be as useless as inconsistent with the limits of this article.