HYGROMETER, or DEW-MEASURER, Is an instrument for measuring the de grew of moisture or dryness of the at mosphere.
Variations in the state of the atmos phere with respect to moisture and dry ness are manifested by a great variety of phenomena ; and, accordingly, numerous contrivances have been proposed for as certaining 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 end less, they may all be referred to two classes ; 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 Ab sorption.—Many substances in each of the three kingdoms of nature absorb moist ure from the atmosphere with greater or less avidity, and thereby suffer some change in their dimensions, or weight, or some of their physical properties. An imal fibre is softened and relaxed, and consequently elongated, by the absorp tion of moisture. Cords composed of twisted vegetable substances are swollen, and thereby shortened, when penetrated by humidity • and the alternate expan sion and shrinking of most kinds of wood, especially when used in cabinet-work, and after the natural sap has been evapo rated, is a phenomenon with which every i one is familiar. Many mineral substan ces 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 re garded as the measure of the quantity of moisture from which the quan tity of water existing in the atmosphere in the state of vapor 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 indica tion 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 foom the absorp tion 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 meteorolo gical observations are attempted. One is
the hair hygrometer of Saussure ; the other the whalebone hygrometer of DeLuc.
Sanssure'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 passed over a grooved wheel or pulley, the axis of which carries an index which moves over a graduated arch. Such is the essential part of the instru ment, and it is easy to conceive how it acts. When the surrounding air becomes more humid, the hair absorbs an addi tional quantity of moisture, and is elon gated ; the counterpoise consequently de scends, 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 di rection. The accuracy of the indications of this instrument depends on the as sumed principle that the expansion and contraction of the hair are due to moisture alone, and are not affected by tempera ture or other changes in the condition of the atmosphere. Experiment shows that the influence of temperature is not very great ; after all precautions have been taken in preparing the instrument, it is found to be exceedingly irregular in its movements, and subject to great un certainties. 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 of whalebone cut trans versely or across the fibres, and stretched by means of a spring between two points. One end is fixed to a bar, while the other acts on the shorter arm of the index of a graduated scale. When the whalebone absorbs moisture it swells, and its length is increased ; as it becomes dry it con tracts ; and the space over which the in dex moves by the one or the other of these effects gives the measure of the ex pansion or contraction, and the corre sponding change in the hygometric state of the atmosphere. The action of this hygrometer appears to be more uncertain than that of Saussure.