H Y G R O M E T R V. 



571 



- - : 

 ' ^ 

 . ' 



: ' 



open at b b, and shut at g. This tube, which is 2 

 inches 8 lines in length, and 1\ lines in diameter, in- 

 ternally, is bored in the direction of its fibres, and re- 

 duced 'by turning it on a lath, till if. thickness is about 

 y^ths of a line, except at the two extremities, where it 

 is left somewhat thicker, to give it greater strength. 

 The piece a add, which is made of brass, connects the 

 ivory with the glass tube, by means of gum mastich, 

 or any other adhesive substance. The part of the ivo- 

 '*bcbc, is protected from the moisture of the 

 air by a brass verrcl, which prevents it from splitting. 

 Before being fitted up, the ivory tube is usually moist- 

 ened on the outside, which may be done very conve- 

 . y, and to a proper degree, by surrounding it with 

 wet cambric, and allowing it to remain till the mois- 

 ture permeates through the ivory. 



1 :. The mercury is introduced by first inserting a 

 hone hair into the bore of the glass tube, sufficiently 

 long to pass completely through it, and reach about an 

 inch beyond its lower extremity, into the ivory tube. 

 A slip of paper, four or five inches long, is then wrapt 

 round the upper part of the glass tube, and tied tightly 

 to it ; leaving about three inches of the paper projecting 

 beyond it, so as to form a kind of funnel for pouring 

 the mercury into the tube. A quantity of mercury be- 

 ing then poured into the paper tube, is afterwards gra- 

 dually made to descend into tlie ivory tube, partly by 

 gentle agitation, and partly by means of the horse hair, 

 which U moved continually upwards and downwards, 

 till the whole of the air is extricated, and the instru- 

 ment filled with the proper quantity of mercury. This 

 being dene, the range of the scale is adjusted in the 

 following manner : The instrument is immersed in a 

 vessel of water, kept at the freezing point, by little bit* 

 of ice floating in it, where it is allowed to remain until 

 the ivory has attained its greatest possible dilatation, 

 which i known by the mercury in the tube becoming 

 stationary. The lowest point which it reachts is then 

 marked as the sera of the scale. One fixed point be- 

 ing thus obtained, the other divisions are determined 

 by previously ascertaining the relation subsisting be- 

 tween the internal capacity of the ivory tube and that 

 of the glass tube, and graduating the instrument ac- 

 cordingly. For this purpose, De Luc employed a glass 

 tube which had formerly been used as a thermometer, 

 the divisions of which he knew, and the bulb of which 

 was intentionally broken, in order that the quantity of 

 mercury cuntaineH in it might be accurately weighed. 

 I be weight of the mercury in the ivory tube being aU 



determined, it was easy to form 



- a I .:]< .vh|.U.l !. the' 

 For let M be the w 

 thermometer, L) 



m the weight of the mercury in the ivory tube, and d 

 the length of a degree upon its scale, we have evi- 

 dently. 



led to the gkas tube and the ivory bulb, 

 e weight of the mercury in the original 

 > the length of a degree upon its scale, 



M 



>: 



1 he scale affixed to 

 vided into as many 



the glass tube was accordingly di- 

 di visions as it admitted, each of 

 which was made equal tod. By this mode of forming 

 the scale, it is imnnessarj to observe, that the instru- 

 ment would be in reality a thermometer, provided the 

 ivory were not affected by moisture ; but the ivory be- 

 tas; dilated and contracted by moisture, will give rise 

 to corresponding deviations from the mere effect of 

 temperature, and thus, by means of an attached ther- 

 mometer , the cfsct due to dryness or moisture may be 

 accurately separated from the rising and sinking of the 



mercury in the glass tube, by means of heat and cold. Hygtome- 



The excess of the hygrometrical degrees above the de- _i?' -. 



grees indicated in similar circumstances b}' an ordinary " Y ^ 

 thermometer, is to be considered as the sole effect of 

 dryness in contracting the ivory ; while a difTrrencc of 

 a contrary kind is to be ascribed to an opposite cause. 



13. Mr Leslie, who has devoted some attention to Mt Leslie's 

 this instrument, has proposed a modification of which, ' vor y h ' r " 

 as he himself remarks, has perhaps carried the hygro-S 10 * 11 * 

 scope thus formed to as high a state of improvement 

 h an imperfect instrument admits. The shell of 

 ivory is turned, in his construction, into an elongated 

 spheroid, about an inch and a quarter in length, and 

 reduced so thin as to weigh only eight or ten grains. 

 At its greatest expansion it contains about SOO grain* 

 of mercury. The upper end, which is adapted to the 

 body by means of a delicate screw, has a slender glas> 

 tube inserted into it, six or eight inches long, and a 

 bore nearly -r T th part of an inch in diameter. The 

 point of extreme humidity is determined, as in the case 

 of De Luc's instniment, by immersing the bulb in wa- 

 ter, or surrounding it with a wet bit of cambric. The 

 divisions of the scale, however, are determined some- 

 what differently : Mr Leslie distinguishes the tube into 

 spaces, which correspond to the thousandth part of the 

 entire cavity, and each of which contains about ,' lh> 

 of a grain of mercury. The ordinary ranjje of tin- 

 scale includes about 7<> of these divisions. The upper 

 extremity of the tube is covered with a small ivory cap, 

 which admits the air, but prevents the escape of tl. 

 mercury, thus rendering the instrument portable. Mi- 

 Leslie remarks, that the contraction of the mercury 

 corresponding to equal increase in the dryncss of th.- 

 air, in M'X times greater at the beginning of the scale 

 than at the 7<>th division ; and that it seems to be in 

 general inversely as the number of hygrometric de- 

 gree*, reckoning from 20 below. He therefore places 

 another scale, on the opposite side of the tube, the in- 

 terval between zero and 70* being divided into 1 00", 

 and corresponding to the unequal portions, from the 

 number 2O to 18') in a logarithmic line, (see Plate 

 CCCXXI. Fig. 5. By extending the logarithmic di- LA , Tl: . 

 visions farther, in conformity with the base of the j! Ct: * L 

 scale, 320 of its degrees would correspond to 108 of 

 the equable divisions, or a contraction of 108 parts in 

 a thousand, with respect to tf . >f tin- hulb. 



At the dry nrs. however, of SOO ot hi- own hygrome- 

 ter, Mr Leslie never (bund the contraction of the ivory 

 to exceed 105. It would have been mure satisfactory, 

 if the temperature at which the observations were made 

 had been given, as we shall afterwards shew, that 300 

 on Mr Leslie '* hygrometer may correspond to very dif- 

 ferent portions of moisture in the medium to which it 

 is exposed. 



I \. The instrument we have described, though very r , - 

 unfit for delicate observations, ro;iy nevertheless be used wlm-li ivory 

 in certain cases with advantage. The slowness of ito li>n>mete 

 indications, when its scale Has once been compared ml * r l * | u ** d 

 with that of a more accurate instrument, is well 

 to point out general results, corresponding to consider- 

 able intervals of time between the observations. Mr 

 Leslie has suggested that, on this account, it may be 

 usefully employed to ascertain the degree of humidity 

 which prevails in the higher regions of the atmosphere, 

 and to determine the hygroscopic state of certain kinds 

 of goods, such ss grain, wool, cotton, Sec. For the lat- 

 ter purpose, all that is necessary is to thrust the intrn- 

 inent among the substances, whose condition, with re- 

 spect to moisture, we wish to determine, and to observe 



