H Y G R O M E T R V. 



573 



(LATIJ 



V .... ..; 



W 

 .. hy- 



The screws n, , only one of which is visible in the re- 

 nreaentation 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 extremity 

 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 arbor ft, 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, 

 ami 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 equa- 

 ble distension by means of a snrnll weight g of three 

 or four grains, which is suspended on the opposite 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 vibra- 

 l iy fixing it in the crayon i, which is intended to 

 receive 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 /. The index o o is fixed to the extremity of the 

 pivot of the arbor _/ e, and points out, by its indications 

 on the graduated arch of the dial plate, the hygrome- 

 tric state of the hair. 



21. SaoMure determined the point of extreme dry- 

 MM, by placing the instrument under a receiver, with 

 a suitable quantity of dry caustic alkali, and allow-in.; it 

 to remiin till the hair ceased to suffer any contra. 

 The point of extreme dry ness thu* determined, though 

 perhap* not absolute, give*, accenting to Saiusure, a 

 degree of drrnen 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 fides of which are kept constantly bedewed with 

 monture. The included air being thus surrounded on 

 all ides with water, become* completely saturated with 

 moisture ; the hair in iu turn it gradually reduced to 

 the nme state, and soon attaint its greatest degree of 

 elongation, so that another fixed point in the scale is ob- 

 tained. If the space described by the index between 

 these two points is greater than a complete revolution, 

 the hair may be shortened till iu length is properly ac- 

 commodated to the range of the scale. The tempera- 

 tare of the air at the time the two extreme point* are 

 determined is of no importance : it may indeed affect 

 the hair thermometncally, in the same manner as it af- 

 fects other substance*, but it produces no change in the 

 hygroscopic indication*. The reason ol t! i- will appear 

 afterwards, when we come to take a theoretical view of 

 the instrument. At present it rosy be sufficient to re- 

 mark, that, in this respect, the hygrometer of SausMtre 

 has a decided advantage over the whalebone hygrome- 

 ter of De I. nc. which, according to the temperature, 

 range* from M* to 100* in air saturated with moisture, 

 where of course it should remain stationary. Sdiisaure 

 divide* his scale sometime* into SfiO, and sometime*) 

 into I >>'". the division* being reckoned, in both case*, 

 from extreme diyma*, which i* assumed for the zero 

 of thr v*rd* extreme moisture, which is mark- 



ed 360. or I Of/ 1 , according to the division adopted. 



?. The principal advantages of Saussure'* hygro- 

 meter are derived. l<t From the unchangeable nature 

 of the material of which it i* formed, by means of which 

 it retain* its hygroscopic power lunger than any other 

 rr;.-." -u 1 ' T mre . '.'<!. From the extreme tenuit) t tin- 

 MMtance jt-.-lf. which enable* it to assume very quick- 

 ly 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 

 shew that hair is totally unfit for hygrometric purpo- 

 ses ; and having very gratuitously assumed the con- 

 tractions 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 hygro- 

 meter must be incorrect, because in certain circumstances 

 they do not coincide with those of his own. He main- 

 tains also, that hair, after it attains its greatest degree 

 of elongation, begins to suffer a contraction, particular- 

 ly if it be allowed to remain beyond a certain time in 

 air saturated with moisture ; but this objection, which 

 is only applicable to hairs of a peculiar structure, may 

 be obvi -ted 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 instruments which have hitherto 

 been formed of organic substances, the hygrometer of 

 Saussure seems the most regular 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 portions of air. The ex- 

 periments of Gay I.uss.10, and the general results he 

 has deduced from them, have given additional value to 

 this instrument, by demonstrating that its expansions 

 are subject to a regular law which admits of analytical 

 investigation. \\ r -hall give an account of these af- 

 terwards. In the meantime, we shall conclude our re- 

 marks on the instrument with a general statement of its 

 indications, compared with those of the whalebone hy- 

 grometer of De Luc. 



This comparative Table of the indication* of the two 

 instrument* was drawn up by De Luc himself: its va- 

 lue, however, is greatly diminished by the temperature 

 being omitted, at which the ob-et v.itions were made. 

 The same remark is applicable to the following obser- 

 of !<'< kiii.m, which differ somewhat from thu-- 

 of De Luc. 



Biimiirf De Luc. 



33 correspond to 10 



54 20 



65 SO 



80 40 



86 45 



22. Some hygrometers are constructed of organic 

 sulxtaiices of a fibroin structure, twisted cither artifi- hygrometer 

 cially or spontaneously during their growth. One of of Hooke. 

 the earliest hygrometers of this description was propo- 



