HYGROMETER

(Gr. vypos, and fierpov, measure.) An instrument for measuring the degrees of moisture, or dryne of the atmosphere. Variations in the state of the atmosphere with respect to moisture and dryne are manifested by a great variety of phenomena, and, accordingly, numerous contrivances have been proposed for ascertaining the amounts of those variations by referring them to some conventional scale. All such contrivances are called hygrometers; but though the variety of form that may be given to them, or of substances that may be employed, is endle , they may all be referred to two cla es; namely, 1st, those which act on the principle of absorption; and, 2d, those which act on the principle of condensation. 1. Hygrometers on the Principle of Absorption. — Many substances in each of the three kingdoms of nature absorb moisture from the atmosphere with greater or le avidity, and thereby surfer some change in their dimensions, or weight, or some of their physical properties. Animal fibre is softened and relaxed, and consequently elongated, by the absorption of moisture. Cords composed of twisted vegetable substances are swollen, and thereby shortened, when penetrated by humidity; and the alternate expansion and shrinking of most kinds of wood, especially when used in cabinet-work, and after the natural sap has been evaporated, is a phenomenon with which every one is familiar. Many mineral substances absorb moisture rapidly, and thereby obtain an increase of weight. Now it is evident that any of these changes, either of dimension or of weight, may be regarded as the measure of the quantity of moisture absorbed, from which the quantity of water existing in the atmosphere in the state of vapour is inferred; but many, indeed the far greater part of them, are so small in amount, or take place so slowly, that they afford no certain indication of the actual state of the atmosphere at any particular moment. Of the different kinds of hygrometers whose construction depends on change of dimensions arising from the absorption of moisture, there are two deserving of notice on account of their historical celebrity, though they are now seldom, if at all, used where accurate meteorological observations are attempted.One is the hair hygrometer of Sau ure; the other the whalebone hygrometer of De Luc. Sau ure's hygrometer consists of a human hair prepared by boiling it in a caustic ley. One extremity of the hair is fastened to a hook, or held by pincers; the other has a small weight attached to it, by which it is kept stretched. The hair is pa ed over a grooved wheel or pulley, the axis of which carries an index which moves over a graduated arch Such is the e ential part of the instrument, and it is easy to conceive how it acts. When the surrounding air becomes more humid the hair absorbs an additional quantity of moisture, and is elongated; the counterpoise consequently descends,and turns the pulley, whereby the index is moved towards the one hand or the other. On the contrary, when the air becomes drier the hair loses a part of its humidity, and is shortened. The counterpoise is consequently drawn up, and the index moves in the opposite direction. The accuracy ot the indications of this instrument depends on the a umed principle that the expansion and contraction of the hair are due to moisture alone, and are not arfected by temperature or other changes in the condition of the ntmosphere. Ex periment shows that the influence of temperature is not very great; but after all precautions have been taken in prep a ring the instrument, It is found to be exceedingly irregular is its movements, and subject to great uncertainties. Besides the substance is soon deteriorated, and will scarcely main tain its properties unimpaired during a single year. The hygrometer of De Luc consists of a very thin slip ol [s. 594]