22. The principal advantages of Saussure's hygrometer are derived, 1st, From the unchangeable nature of the ma terial of which it is formed, by means of which it retains its hygroscopic power longer than any other organic sub stance ; 2d, From the extreme tenuity of the substance it self, which enables it to assume very quickly the state of the surrounding medium ; and, 3d, From the little effect which, in consequence of this tenuity, it produces on the hygroscopic state of the medium to which it is exposed. De Luc has endeavoured, more it would appear from a desire to recommend his own instrument than to promote the cause of science, to show that hair is totally unfit for hygrometric purposes ; and having very gratuitously as sumed the contractions and expansions of whalebone as a standard of reference for other hygroscopic substances, he has taken it for granted, that the indications of Saussure's hygrometer must be incorrect, because in certain circum stances they do not coincide with those of his own. He maintains, also, that hair, after it attains its greatest degree of elongation, begins to suffer a contraction, particularly if it be allowed to remain beyond a certain time in air satu rated with moisture ; but this objection, which is only ap plicable to hairs of a peculiar structure, may be obviated by attending to the directions of Saussure, who was aware of the fact, and particularly enjoined, that such hairs as retrograded more than 2° should be rejected as unfit for the construction of hygrometers. On the whole, we have no hesitation in saying, that of all the hygroscopic instru ments which have hitherto been formed of organic sub stances, the hygrometer of Saussure seems the most regu lar in its elongations and contractions ; the least liable to be affected by exposure to the weather ; and the best adapted for ascertaining the hygrometric state of small por tions of air. The experiments of Gay Lussac, and the general results he has deduced from them, have given ad ditional value to this instrument, by demonstrating that its expansions are subject to a regular law which admits of analytical investigation. We shall give an account of these afterwards. In the mean time, we shall conclude our re marks on the instrument with a general statement of its indications, compared with those of the whalebone hygro meter of De Luc.
This comparative Table of the indications of the two in struments was drawn up by De Luc himself: its value, however, is greatly diminished by the temperature being omitted, at which the observations were made. The same remark is applicable to the following observations of Bock man, which differ somewhat from those of De Luc.
22. Some hygrometers are constructed of organic sub stances of a fibrous structure, twisted either artificially or spontaneously during their growth. One of the earliest hygrometers of this description was proposed by the cele brated lIooke, and formed of the heard of the wild oat, which twists and untwists itself according to the state of the air with respect to moisture. The heard is fixed atone i extremity, and an index being applied at the other in a transverse position, its motions along the graduated cir cumference of a circle point out the hygroscopic state of the air.
23. Upon a similar principle Mr Molyneux proposed, in 1685, to construct an hygrometer of whip cord or cat gut, by suspending it from a hook, with a small weight at the lower extremity to give it a proper degree of tension, and early at the same time an index over a graduated cir cle described on a fixed board below. 'We shall give a
description of the instrument in a modified form, recom mended by Mr Coventry of Southwark. AB, Fig. 7. re presents the cat-gut, which may be of any convenient length. It is suspended from the bracket AD, and stretched by the weight F at its lower extremity. At B is a circular card of pasteboard attached to a round bit of cork, through which the cord is made to pass. The circumference of the card is graduated into one hundred equal parts. Ano ther card, connected with the cat-gut in a similar manner, and intended to record the revolutions of the other, is placed at C, at one-tenth of the length of AB from the fixed point A, and divided into ten equal parts. DE is a vertical line, along the frame which supports the cat-gut, and serves to point out the indications of the circular cards. In adjusting the instrument to extreme moisture, the cord is completely moistened with water, and when it ceases to untwist itself, both the circular cards are turned round till the zero upon each points to the vertical line Dli. It is more difficult to obtain another fixed point ; and indeed this is not very necessary, as the instrument, though it possesses great sensibility, can scarcely be used for any purpose but to point out general differences with respect to moisture.
24. The increase of weight, which all hygroscopic sub stances acquire by the absorption of moisture, furnishes another general principle for the construction of hygrome ters. But as the accuracy of these instruments is liable to be gradually affected by changes in the hygrometric property of the substances themselves, as well as by the deposition of dust and other light bodies on their surfaces, few hygrometers have been constructed on this principle. The substances usually employed for the purpose are, sul phuric acid, the deliquescent salts, and paper. The first of these substances was recommended by Mr Gould so early as 1684, who observed that sulphuric acid, after ab sorbing a certain portion of water from the atmosphere, continued to retain it till the air became drier, when it again yielded up a portion of the moisture it had previously ac quired ; and these alternations of the absorption and extri cation of moisture always corresponded to the hygroscopic state of the air. He therefore placed a quantity of the acid in a cup in one scale of a balance, and a counterpoise in the other ; and ascertained the relative state of the air with respect to moisture or dryness, according as either arm of the balance preponderated. A contrivance of this kind answers well enough for poiating out general results, but it is totally unfit fur discovering the hygroscopic state of the air at any particular instant of time. We hasie found, however, that paper, soi.ked in a weak solution of the mu nate of lime, and then dried, is very rapid in its indications, and capable of being affected by very minute changes; and indeed we have reason to think, from the obsei rations we have made with a small slip of paper prepared in that manner, and suspended front one of the arms of a delicate balance, that an hygrometer constructed upon this princi ple might be obtained, possessing the utmost sensibility. Other hygrometers of a similar construction have been employed, but they are totally unworthy of notice.