H Y G R O M E T R Y. 





- aed by the celebrated Hooke, and formed of the beard 

 "7- of the wild oat, which twists and untwists itself accord- 

 "" """ ' ing to the state of the air with respect to moisture. The 

 beard is fixed at one extremity, and an index being ap- 

 plied at the other in a transverse position, its motions 

 along the graduated circumference of a circle point out 

 the hygroscopic state of the air. 



fat-gut hy 23. Upon a similar principle, Mr Molyneux propo- 

 gnmietcr of sed, in 1685, to construct an hygrometer of whip cord or 

 Molyncux cat-gut, by suspending it from a hook, with a small 

 Vcn ~ weight at the lower extremity to give it a proper degree 

 of tension, and carry at the same time an index over a 

 ., .. graduated circle described on a fixed board below. We 

 .; ^ shall give a description of the instrument in a modified 

 form, recommended by Mr Coventry of Southwark. 

 AB represents the cat-gut, which may be of any conve- 

 nient 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 100 

 equal parts. Another card, connected with the cat-gut 

 in a similar manner, and intended to record the revolu- 

 tions of the other, is placed at C, at one-tenth of the 

 ength of AB from the fixed point A, and divided into 

 10 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 in- 

 strument 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 

 nero upon each points to the vertical line DE. 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. 



Ba'ance by- 2 *' Tne > ncrease of weight, which all hygroscopic 



gromcters. substances acquire by the absorption of moisture, fur- 



nishes another general principle for the construction of 



hygrometers. But as the accuracy of these instruments 



is liable to be gradually affected by changes in the hy- 



grometric 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 sulphuric acid, the deliquescent 



salts, and paper. The first of these substances was re- 



Siilplmric commended by Mr Gould so early as 1684, who obser- 



ncid recom- ved that sulphuric acid, after absorbing a certain por- 



mended by tion of water from the atmosphere, continued to retain 



Gould. ; t till the air became drier, when it again yielded up a 



portion of the moisture it had previously acquired ; 



and these alternations of the absorption and extrication 



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 counter- 



poise ,'n 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 contri- 



vance of this kind answers well enough for pointing 



out general results, but it is totally unfit for discover- 



ing the hygroscopic state of the air at any particular in- 



Paper soak- slant of time. We have found, however, that paper, 



cd in muri- soaked in a weak solution of the muriate of lime, and 



*e of lime, 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 observations we have 



made -with a small slip of paper prepared in that man- 





ner, and suspended from one of the arms of a delicate Hygwmi- 

 balance, that an hygrometer constructed upon this prin- 

 ciple might be obtained, possessing the utmost sensi- 

 bility. Other hygrometers of a similar construction 

 have been employed, but they arc totally unworthy of 

 notice. 



25. Mr Leslie has proposed an hygrometer, totally Mr T ' es: 

 different in principle from any of those we have consi- ; 

 dered, but perhaps superior to all of them, both in 

 point of accuracy and delicacy. Hygrometers formed 

 of organic substances are liable to be affected by the 

 partial decompositions, which, by exposure to air and 

 moisture, such bodies continually undergo; and though 

 some of them are composed of materials which resist 

 the action of the weather better than others, none of 

 them can be said to be indestructible, and all of them, 

 in the course of time, lose in a great degree their hy- 

 groscopic properties. Their scales, therefore, however 

 accurately constructed at first, are subject to a gradual 

 derangement, and require occasional adjustments to 

 render their indications at all correct. This is certain- 

 ly a great objection to the use of these instruments ; 

 but it is an objection from which the hygrometer of 

 Mr Leslie is entirely free, and as we have derived a 

 formula by which the absolute quantity of moisture 

 contained in a given volume of air may be accurately 

 determined, in terms of the degrees of its scale, we 

 must now consider it as by far the most accurate hy- 

 grometer that has yet been proposed. The instrument PLATE 

 consists of two spheres of glass A, B, connected with ccCXXli 

 each other by a bent tube CDEF, which is fixed to the Fig- 8 - 

 stand GH, and contains inclosed a small portion of sul- 

 phuric acid, tinged with carmine to render it more dis- 

 tinctly-visible. When the spheres, both of which are 

 filled with air, are at the same temperature, the liquor 

 in the recurved tube remains stationary ; but if one of 

 the balls, as A, be colder tlian the other B, the air in 

 the latter, by its greater elasticity, immediately de- 

 presses the liquor in the limb FE, and raises it in an 

 equal degree in the limb CD. One of the balls is ac- 

 cordingly covered with a coating of cambric, or tissue 

 paper, and kept continually moist with pure water, con- 

 veyed to it by filaments of floss silk from an adjoining 

 vessel. The evaporation of the water quickly cools 

 the surface of the ball, in a degree proportioned to the 

 rapidity with which the process is carried on, which 

 will depend partly upon the temperature, and partly 

 upon the dryness of the ambient medium ; and hence 

 the depression of the liquor in the limb FE becomes an 

 indication of the relative dryness of the surrounding 

 air. The caloric abstracted from the moistened ball by 

 evaporation, is incessantly supplied by the air and the 

 contiguous bodies, and iii the course of two minutes the 

 maximum of effect is produced. Were it not for this 

 continual influx of temperature, no limits could be as- 

 signed to the degree of cold that might be induced. 

 Tiie scale is formed by dividing the interval between Graduation 

 the boiling and freezing points into 1000 equal parts, of the scale. 

 so that 10 correspond to 1 of the centigrade thermo- 

 meter, and 50 to 9 of Fahrenheit. This hygrometer 

 acts equally well when the moisture on the balls is in a 

 frozen state ; but the heat required for the melting of 

 ice being about a seventh part of what is necessary for 

 the conversion of water into vapour, the temperature of 

 the coated ball will, in like circumstances of the air 

 with respect to moisture, sink a seventh part more than 

 before ; and therefore the degrees indicated by the in- 

 strument must, in that case, be reduced 1 in 7, to 

 adapt the scale to the actual state of things. 



