5. Both animal and vegetable substances of a fibrous structure, possess the property of being dilated by mois ture, in a direction transverse to the fibres ; and, accord ingly, the lateral expansion of these bodies furnishes the principle upon which a considerable number of hygrome ters are constructed. One of the earliest hygrometers of this kind was proposed by Mr Conicrs in 1676. The whole contrivance is of the rudest kind ; and though it is scarcely worthy of notice, we shall give a brief description of it, in order that some idea may be formed of the imperfect state of hygrometry at that period. AAAA (Plate CCCXXV. Fig. 1.) represents a frame of wood grooved in the inside for admitting two pannels of deal B, 11 to play freely at top and bottom. The pannels, which arc placed so as to have their fibres in a vertical po sition, are fastened to the frame at each side, and a suffi cient 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 approach ing nearer to it, gives a circular motion to the index itself, by means of a slender metallic chain, which passes round the axis, and is fastened to one of the pannels at F. AN' is a weight or counterpoise connected with the axis by means of a string passing over the pulley D, and attached to the aTm CG, and which causes the index to descend, as the pannels expand by moisture. Several other contrivances of a similar kind are described in the Philosophical Trans actions for that period, hut they are all equally rude and imperfect.
6. The most accurate hygrometer, constructed on the principle of a lateral xpanston, is that of Dc Luc. The substance he employs in preference to all others, is whale bone cut transversely into thin slips. Such is the tenacity of this substance, that, according to Dc Luc, these slips may be a foot long, and a line in breadth, Nl it h out weighing above one-fourth of a grain, and yet be capable of support ing a weight of about 160 grains. The instrument is fitted up in various forms by (fillet eta artists; but the general princi ples of its construction are nearly the same in all. Mr Adie, Edinburgh, constructs it in the following manner. ABC D, Fig. 2.. represents the frame-work of the instrument, to the upper part of which is attached the graduated circle EF, capable of being elevated or lowered at pleasure, to suit the length of the slip of whalebone-a b. The whale bone, which is usually about 10 inches in length, is fasten ed at a between two slips of brass by means of a screw, and in a similar manner at b, where it is connected with a slen der silk hand which moves over the axis, to the extremity of which the index is fixed ; and it is kept in a state of uniform distension, by a silk thread passing round the axis,(to which it is fixed,) in a direction contrary to that of the silk hand, and fas-,c net] at the lower ext•emiI to a spiral gilt silver wire c. This spiral spring forms, by its reaction, an excellent countet poise to the whalebone, as it acts with the smallest energy when the latter is most weakened by dilatation.
7. The xneme points of the scale, by which the inter Divisions are graduated, are determined in the fol bra irg I:, Amer : To obtain the point of extreme humidity, or complete saturation with moisture, De Luc is not satis !led u,ith exposing the instrument to a portion of air per fectly saturated with the vapour of water, but he actually immerses it in that fluid, and allows it to remain in it, till it teases to suffer any farther dilatation. The point on the
circular sc..le, to which the index reaches, is then marked as cxtreme moisture. The opposite point, namely, that of extreme dryness, is obtained by enclosing the instrument untier a receiver with a quantity of dry quicklime, and al lowing it to remain exposed to its action, till the whale bone attains its greatest degree of contraction, which is generally in about three weeks. The quicklime, by its at traction for moisture, gradually absorbs the watery vapour contained in the air, which, in its turn, abstracts moisture from the whalebone, till an equilibrium is established he tweet' the attraction of the two substances for vapour. As the whalebone becomes drier, its fibres continually ap proach toss a rd s each other ; and at last, when it ceases to yield any moisture, it also ceases to suffer contraction, and the index points to extreme dryness. The length of the slip of whalebone is so proportioned to the diameter of the axis, or arbor to which the index is fixed, that the interval between the points of extreme moisture and dryness em braces a range which is somewhat less than a complete re volution ; and this interval is then divided into 100 equal parts. The zero of the scale is usually marked at ex treme dryness, and the divisions are in that case numbered upwards to extreme moisture, which is marked 100. Sonic artists, however, reverse this order, and place the zero at extreme moisture,—a practice, which cannot fail to lead to mistakes in recording the indications of the instrument.
7. Saussure, to whom we are indebted for many useful and interesting observation on hygrometry, has stated several objections against this instrument, applicable both to the substance of which it is formed, and the manner of determining the extreme points of the scale. He affirms, that whalebone, being a substance of a muscular or gela tinous nature, would admit of an indefinite relaxation by moisture, were it not for certain filaments, which connect the fibres with each other, but do not prevent them from separating beyond the limits, to which their hygroscopical affinity for moisture in the vaporous state would dilate them. He maintains, therefore, and we think with justice, that to immerse such a substance in water, in order to obtain the greatest relaxation of which it is susceptible, is to reduce it to a condition which it can never after wards arrive at by the it of vapour ; and quently, that all the divisions of the scale, which are in cluded between the perfect humidity of the whalebone, and the point corresponding to the complete saturation of air with moisture, arc entirely useless. He adds, that if whalebone is to be used at all in the construction of hygro meters, the point of extreme moisture should be marked probably where 80° stands in De Luc's scale.