18. An hygrometer depending on the principle of a ge neral expansion by moisture, and fitted up in the same man ner as the hygrometer of De Luc just alluded to, has been strongly recommended by Jean Baptiste, a Capuchin friar of St Martin at Vicenza. The hygroscopic substance used in the construction of this instrument is a narrow slip of the allantois of a calf, the thin membrane which enve lopes the fcctus of animals before birth. The point of ex treme moisture was fixed by exposing the instrument to air saturated with aqueous vapour. Another point was determined by heating to the temperature of 50° of Reau mur, or 144 ° of Fahrenheit, a small stove, which was kept open, and preserved for some time as nearly as possible at the same temperature, and then introducing the instrument into it, where it was allowed to remain so long as the allan tois suffered contraction. According to Jean Baptiste, the degree of dryness obtained by this process is fixed and in variable. The intermediate space on the scale, between the extreme points thus determined, was divided as usual into 100 equal parts. In a subsequent part of this article we shall demonstrate, that no fixed point can be obtained for the graduation of an hygrometer, by a mere exposure of the instrument to an elevated temperature ; and, conse quently, the scale of this hygrometer can no more be relied upon than that of Chiminello. The substance it self, however, we have reason to think, from the experi ments we have made with it, is exceedingly fit for hygrome tric purposes.
19. But of all the hygroscopic instruments which we have hitherto described, and which al e constructed on the principle of a general, or at least of a longitudinal ex pansion, the hygrometer of Saussurc is by far the most de licate, as well as the most accurate and uniform, in its indi cations. The substance which he selected, in preference to every other, was a human hair, the elongations and con tractions of which by moisture and dryness, though less extensive than some of the substances already mentioned, may be rendered sufficiently sensible by mechanical con trivances. As the value of this instrument has been greatly enhanced by the late researches of Gay Lussac, a particu lar description of it is the more necessary : we shall there for e give an account of the different parts of it in detail.
20. The general appearance of the instrument is nearly the same as that of the whalebone hygrometei of De Luc, which seems indeed to have been borrowed from it, the pri ority of invention being due to Saussure. The upright pil lars a a a a, Fig. 4, which support the dial-plate, are fixed to the rectangular frame b b b b, at each corner of which is a screw for fixing the instrument to the bottom of its case, when it is exposed to the external air. The dial-plate is made to slide along the pillars a a, and is thus capable of being raised to any particular altitude, in order to suit the length of the hair. The screws n, n, Fig. 6. only one of which is visible in the representation of the instrument, are intended to fix the dial-plate after it has been raised to the proper height. The lower extremity of the hair is held fast by the chops of the screw pinions d, and the upper ex tremity of it is fixed in a similar manner at c. The pinions c
also serve to connect the hair with a thin slip of silver which is rolled round the arborf e, and fixed to it. The part of the arbor on which the silver thread is wound is usually cut in the form of a spiral groove, and made quite flat at the bottom, in order that the slip of silver may be always at the same distance from the axis of the arbor. The hair is kept in a state of equable distension by means of a small weight g of three or four grains, which is suspended on the oppo site side of the arbor, and is sufficient to stretch the hair without breaking it. When the instrument is transported from place to place, the weight g is prevented from vibrat ing, by fixing it in the crayon i, which is intended to re ceive it, and in which it is securely fixed by means of the screw k. The crayon itself is moveable along the bar h h, and may be fixed in any position by the screw 1. The in dex o o is fixed to the extremity of the pivot of the arbor f e, and points out, by its indications on the graduated arch of the dial plate, the hygrometric state of the hair.
21. Saussure determined the point of extreme dryness, by placing the instrument under a receiver, with a suitable quantity of dry caustic alkali, and allowing it to remain till the hair ceased to suffer any contraction. The point of extreme dryness thus determined, though perhaps not absolute, gives, according to Saussure, a degree of dry ness which is perfectly fixed and uniform. The point of extreme moisture is obtained, by placing the instrument over water in the inside of a receiver, the sides of which are kept constantly bedewed with moisture. The included air being thus surrounded on all sides with water becomes completely saturated with moisture ; the hair in its turn is gradually reduced to the same state, and soon attains its greatest degree of elongation, so that another fixed point in the scale is obtained. If the space described by the in dex between these two points is greater than a complete revolution, the hair may he shortened till its length is pro perly accommodated to the range of the scale. The tem perature of the air at the time the two extreme points are determined is of no importance : it may indeed affect the hair thermometrically, in the same manner as it affects other substances, but it produces no change in the hygro scopic indications. The reason of this will appear after wards, when we come to take a theoretical view of the in strument. At present it may be sufficient to remark, that, in this respect, the hygrometer of Saussure has a decided advantage over the whalebone hygrometer of De Luc, which, according to the temperature, ranges from to 100° in air saturated with moisture, where of course it should remain stationary. Saussure divides his scale some times into 360°, and sometimes into 100°, the divisions be . ing reckoned, in both cases, from extreme dryness, which is assumed for the zero of the scale, towards extreme moisture, which is marked 360°, or 100°, according to the division adopted.