570 



HYGROMETRY. 



Hyijrotne. screw, and in a similar manner at b, where it is con- 

 nected with a slender silk band which moves over the 

 axis, to the extremity of which the index is fixed ; and 

 rrxl '* ' s kept m a state ^ un ''' orm distension, by a silk 

 Tig. t.~ ' thread passing round the axis, (to which it is fixed,) in 

 a direction contrary to that of the silk band, and fasten- 

 ed at the lower extremity to a spiral gilt silver wire d e. 

 This spiral spring forms, by its reaction, an excellent 

 counterpoise to the whalebone, as it acts with the small- 

 est energy when the latter is most weakened by dilata- 

 tion. 



Method of 7. The extreme points of the scale, by which the in- 

 obtaining termediate divisions are graduated, are determined in 

 the extreme t ij e following manner : To obtain the point of extreme 

 Kall" f the humidity, or complete saturation with moisture, De Luc 

 is not satisfied with exposing the instrument to a por- 

 tion of air perfectly saturated with the vapour of water, 

 but he actually immerses it in that fluid, and allows it 

 to remain in it, till it ceases to suffer any farther dilata- 

 tion. The point on the circular scale, to which the in- 

 dex reaches, is then marked as extreme moisture. The 

 opposite point, namely that of extreme dryness, is ob- 

 tained by inclosing the instrument under a receiver 

 with a quantity of dry quicklime, and allowing it to re- 

 main exposed to its action, till the whalebone attains its 

 greatest degree of contraction, which is generally in 

 about three weeks. The quicklime, by its attraction 

 for moisture, gradually absorbs the watery vapour con- 

 tained in the air, which, in its turn, abstracts moisture 

 from the whalebone, till an equilibrium is established 

 between the attraction of the two substances for vapour. 

 As the whalebone becomes drier, its fibres continually 

 approach towards 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 embraces a range which is some- 

 what less than a complete revolution ; and this interval 

 is then divided into 100 equal parts. The zero of the 

 scale is usually marked at extreme dryness, and the di- 

 visions are in that case numbered upwards to extreme 

 moisture, which is marked 100. Some artists, however, 

 reverse this order, and place the zero at extreme mois- 

 ture, a practice, which cannot fail to lead to mistakes 

 in recording the indications of the instrument, 

 abjections 7. Saussure, to whom we are indebted for many use- 

 to the use of f u i an( j interesting observations on hygrometry, has 

 whalebone sta t e( j several objections against this instrument, appli- 

 poof P hyi cable both to the substance of which it is formed, and 

 groinetry. the manner of determining the extreme points of the 

 scale. He affirms, that whalebone, being a substance of 

 a muscular or gelatinous nature, would admit of an in- 

 definite 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, there- 

 fore, and we think with justice, that to immerse such a 

 substance in water, in order to obtain the greatest relax- 

 ation of which it is susceptible, is to reduce it to a condi- 

 tion which it can never afterwards arrive at by the influ- 

 ence of vapour ; and consequently, that all the divisions 

 of the scale, which are included between the perfect hu- 

 midity of the whalebone, and the point corresponding 

 to the complete saturation of air with moisture, are en- 

 tirely useless. He adds, that if whalebone is to be used 

 at all in the construction of hygrometers, the point of 



extreme moisture should be marked probably where 80 s Hygtome- 

 stands in De Luc's scale. v _- ry '- 



8. With respect to the use of dry quicklime as a de- 

 siccative, Saussure seems to suspect that this substance ti^ubstan" 

 produces a degree of dryness less perfect, than the fixed C es employ, 

 alkalies in a caustic state ; and that at any rate, the slow- ed to obtain 

 ness with which it attracts moisture, renders it less fit the P oint of 

 for the purpose. Strongly concentrated sulphuric acid, j*' 

 or paper soaked in muriate of lime, and then well 

 dried, absorb moisture more rapidly than either : the 

 former, in particular, when inclosed under a receiver 

 with a given volume of air, causes the index of a hy- 

 grometer to advance as many degrees towards extreme 

 dryness, in a few hours, as dry quicklime would do in 

 as many days. As the attraction of sulphuric acid for 

 moisture varies, however, with its degree of concentra- 

 tion, it is proper to employ it always of the same spe- 

 cific gravity, otherwise the point of extreme dryness 

 will not be the same in all hygrometers. An unifor- 

 mity of scale would thus be obtained, whether the 

 point of extreme dryness were absolute, or merely re- 

 lative. 



9- For examining the hygroscopic state of small De Luc' 

 quantities of air, De Luc gives the instrument a form, hygrometer 

 which renders it more commodious for being intro- ^ "j^jj 

 duced under a receiver. Fig 3- represents a front p0 jf io s of 

 view of the instrument constructed for that purpose, of air. 

 its actual size, a a a a is a frame of brass, which is PLATE 

 connected with a similar frame behind. The dotted ccoxxi. 

 line ebb b d represents the slip of whale bone fixed at Fig. 3. 

 c to an adjusting screw, passing over the pullies bbbb, 

 and joined at d to a slender metallic plate of annealed 

 silver. This metallic plate d ef moves over a pulley e, 

 and is joined at/ to the moveable part g of the vernier 

 h h h h. From the top of the moveable vernier i pro- 

 ceeds another slender metallic plate of the same pliant 

 material, the opposite extremity of which is fixed to 

 the pulley k. This pulley is supported on the same 

 axis with a smaller pulley /, which is connected, by 

 means of a slender metallic plate, with the upper ex- 

 tremity of the bent, lever m, n, the shorter arm of which 

 n is pressed by a spring. It is easy to see from the 

 figure of the instrument, that when the whale-bone is 

 dilated by absorbing moisture, the vernier will ascend 

 by the action of the spring upon the lever ; and vice 

 versa. 



10. The instrument may be fitted up in the form of Pocket hy- 

 a watch, by fixing the whale-bone to the circumference grometen. 

 of a wheel, and distending it gently by a weak spring. 



The contraction and enlargement of the whale-bone 

 might then be indicated by a hand fixed to the arbor of 

 another wheel, and moving along the circular gradua- 

 tion of a dial-plate. Other contrivances of a similar 

 kind will readily suggest themselves to the ingenious 

 artist. 



11. Hygrometers have also been constructed of sub- DeLue's 

 stances, which seem to suffer a dilatation in all direc- 1Tor ? ^3**- 

 tions, by moisture. On this principle, De Luc con- m 

 structed some time ago a hygrometer of ivory, by gi- 

 ving a portion of that substance intended to be affected 



by moisture, the form of a thin slender tube, and then 

 inserting into it, at one extremity, a capillary tube of 

 glass, about 14 inches long, and -^ inch in diameter. 

 The ivory is recommended to be taken from an ele- 

 phant's tooth of considerable size, a few inches from 

 the top, and as near the surface as possible, in order 

 that it may be of an uniform grain in different instru- 

 ments. This hygrometer is represented in Fig. 4. Fig. 4. 

 Plate CCCXXI. where bgb represents the ivory tube 



