MYiiROMKTUY. 



HTQBOMETBY. 







respect to tension, it will be pr<>| the following circum- 



liili in : When an hygrometer, like that of Siwissure or I)o Luc u 

 introduced into clow vassal, or into any pert of apace fully saturated 

 with aqueoua vapour, it is observed that, whatever be the t-iii|T.itni.-. 

 the index point* to the aame degree ; from which it may be inferred 

 that equal quantities of vapour hare been absorbed by the material 

 (hair or whalebone) of which the instrument consists, notwithstanding 

 the difference of temperature. In fact the vapour in the veaeel, or 

 pace, U in such a state that the presence of a material having the least 

 possible attraction for water is sufficient to produce a precipitation of 

 the Utter the hygroscopic material has an attraction for water, and 

 thus it absorbs that which is precipitated. But the quantity absorbed 

 U so small as not to tininili sensibly the elasticity of the vapour in the 

 Timnl ; and therefore the absorption continues to the full extent of the 

 attraction of the material : the quantity thus absorbed is necessarily 

 Constant, unless the attraction undergo some change by a change of 

 temperature ; but experience proves that the attraction of the material 

 is not sensibly altered by such change within the usual limit* of the 

 thermorortric scale. 



When the vessel into which the hygrometer is introduced is not 

 completely saturated with water, the quantity of water absorbed by the 

 hygroscopic material is limited by the power of the latter to absorb the 

 precipitated moisture : that power diminishes in proportion to the 

 quantity received, so that the attraction of the material for water 

 i fsism to act when it U equivalent to the pressure which the vapour 

 can support without becoming liquid ; and the elongation of the hair 

 or whalebone then ceases, or the index remains stationary on the scale. 



In order, then, to determine the law according to which the attrac- 

 tion of the hygroscopic material for water diminishes as the precipi- 

 tated water U absorbed by it, or, in other words, to find on the scale of 

 the hygrometer a number of points corresponding to any given elasti- 

 cities of the vapour, Oay Lussac put water into a vessel of glass ; and, 

 having determined the elasticity of the vapour arising from it, he 

 suspended from the upper part of the vessel a delicate hygrometer of 

 the kind invented by Saussure. The vessel was then closely covered, 

 so that there might be no communication between the vapour within 

 and the external air ; when, after a short time, the index of the hygro- 

 meter became stationary at a certain point on the circumference of the 

 graduated ring which served as a scale ; this point thus became on 

 indication of the elasticity of the vapour. Experiments of the like 

 kind being made with vapour of equal temperature, but in different 

 states of elasticity, between those which correspond to extreme drynegs 

 and complete saturation, there were obtained so many points on the 

 scale of the instrument as indications of the elasticities of the vapour. 



From the results of these experiments, M. Biot found, by interpo- 

 lation, a table of the elasticities of vapour for every degree of Saussure's 

 hygrometer, the temperature being 10* of the centigrade thermometer 

 (50* Fahr.). He also formed a table showing the degrees of the hygro- 

 meter corresponding to every degree of elasticity. The extremes 

 of dryness and moisture on the scale, and also the corresponding 

 extremes of elasticity, were indicated respectively by and loo . l;i.,t . 

 'Traite' de Physique,' liv. i. ch. 13); but the elasticities or tensions 

 would be more conveniently expressed in terms of the elasticity at the 

 point of complete saturation, which in then represented by unity. 



The numbers in the table are formed from the observed tensions at 

 a constant temperature equal to 50* (Fahr.) ; and it might be supposed, 

 since the index of the hygrometer stands constantly at 100" when the 

 material is acted on by vapour in the state of maximum tension 

 whatever be the temperature, that the index should stand at one point 

 on the scale when the tensions of the vapour have the same proportion 

 to the maximum tension at their respective temperatures : this suppo- 

 sition is not quite correct ; but it may be presumed that in using 

 Blot's table for temperatures differing from 60* Fahr., the error in the 

 tendons win not be considerable. 



Oay Lussac having proved that vapours, whether those of pure 

 water or those of different liquids intermixed, while they retain their 

 character of elasticity, suffer the same variations of volume by variat ions 

 of pressure as are suffered by fluids, which are permanently elastic 

 within ordinary ranges of temperature and pressure, determined, by 

 subsequent experiments, the volumes of the vapour produced by a given 

 weight of water at given temperatures and under given atmospherical 

 pressures; and thus, consequently, obtained the quantity of m 

 in a given volume of vapour. The results of his experiments were 

 reduced to a formula by Biot ; and subsequently, with certain modifica- 

 tions, to one in English weights and measures by Dr. Ander- 

 writer of the article on Hygrometry in the ' Edinburgh Encyclopaedia.' 

 This formula i* 



10958 n.r 



II7-I-H ' 



in which o i, in grains, the quantity of moisture in a cubic inch of 

 vapour at the temperature represented by ( (Fahrenheit's scale), r is 

 the elastic force of the vapour at the same temperature, and is the 

 height of the barometrical column in inches at the time of the experi- 

 ment.' It agrees nearly with that which was obtained by Dal ton from 

 experiment* on the state of the thermometer at the dew- point, the 

 height of UM mercurial column in that result being 80 inches. Km,,, 

 tWsformuk, the temperature (being 60* (Fahr.), B= SO inches, and 



r = 0-8/5 (from Dalton's table of the elastic force of vapour corre- 

 ponding to that temperature and that density of the air), we 

 have o - -002477, the grains of moisture in a cubic inch 

 vapour. 



The value of o being thus found for any given states of the baro- 

 meter and thermometer; the weight of moisture, in grains, in .. 

 inch of air of the like density and temperature, and corresponding to 

 any observed degree of Sauseure's hygrometer may be obtained on 

 multiplying that value by the number in Biot's table corresponding to 

 the observed degree and dividing the product by 100 ; this ii . 

 must be made because, in that table, 100 represents the clout 

 the vapour when in the state of complete saturation. 



The extreme points on the scale of an hygrometer acting by the 

 elongation of o material, like those of Saussure and De Luc, may be 

 found in the following manner : the instrument is to be placed under 

 a receiver in which is a certain quantity of dry caustic alkali ; when, 

 after a time, the material will contract in length as much as its 

 nature will permit ; the point on the scale at which the index stands is 

 that of extreme dryness, and constitutes the zero point. The instru- 

 ment may then be placed in water, or in a receiver filled with vapour 

 completely saturated with moisture, when the material will expand to 

 the greatest extent possible : the place of the index is then to be con- 

 sidered as the point of extreme humidity, and is usually indicated by 

 100. The employment of organic substances in hygronietry, has how- 

 ever, been long since almost disused, for exact purposes. 



Leslie's hygrometer consuls of a glass tube bent so as to form two 

 equal branches parallel to one another, and each terminating with a 

 hollow ball into which is introduced sulphuric acid, coloured. One of 

 the bolls is covered with cambric, which is kept constantly moist by 

 water from a neighbouring vessel ; and the evaporation of the water, 

 by cooling that ball, allows the air in the other, by its superior 

 elasticity, to depress the acid in the tube below and force it to rise in 

 the other. The degree of evaporation depends partly on the tem- 

 perature, and partly on the state of the surrounding atmosphere with 

 respect to humidity ; and hence the depression of the acid in the tube, 

 being measured by a convenient scale, affords an indication 

 relative dryness of the air. In order to determine the absolute 

 quantity of moisture in a given volume of the atmosphere liy the 

 state of his hygrometer, Leslie, having found from some experiments 

 that the capacity of air for caloric was j of that of water, and having 

 ascertained that the quantity of caloric necessary to convert a given 

 volume of water into vapour was expressed by 6000 degrees of his 

 instrument; concluded that the same quantity of caloric won], 

 an equal volume of air to a temperature expressed by j x 8000, or 

 16,000 degrees of the instrument ; and consequently that, at the tem- 

 perature of the wet ball, atmospheric air contains a quant 

 mi."ture equal to t( jgg part of its weight for each degree; the scale 

 between the points of extreme dryness and extreme in.. 

 divided into one thousand parts. (Treatise ' On tlir Kdati 

 Heat and Moisture.') 



From the fact that the elastic forces of pure vapour and of vapours 

 mixed with atmospheric air are equal to one another, the expansion 

 which air undergoes in consequence of being saturated with in< 

 may be found. For if v represent a given volume of dry air, v' the 

 volume when saturated, and B, in inches, the height of the baron i 

 column ; then the elastic force of the air, under the increased \ 



v' is \1?- Now F representing the clastic force of the vapour in inches 



of mercury, which, for the given tcni]>er.-iturc. may be found from 

 D.iltonV table, the Mini of the elastic forces of the air and vapour will 



be expressed by r + -; and this being made equal to B, the pressure 



of the atmosphere, the value of v'-v maybe found. Making v 1. 

 that value expresses the expansion in a fractional part of the volume of 

 dry air. 



At any place on the surface of the earth, the mean temperature at 

 which moisture begins to form in the atmosphere may be ; 

 Dalton's formula E = J,II (F-F'), in which E is the number of grains 

 evaporated in one minute from the surface of water in a i-yln 

 vessel 6 inches in diameter and 1 inch deep, F is the da 



!ii the atmosphere at o given t. nip. r.iture, which may be the 

 mean annu.il t. !, ipcr.it HIV .t tin' place (50 lain., 

 and v' the elastic force exerted at the time that the moisture bee 

 form : U is, in grain-, the evajoratiiig force in an atmosphere supposed 

 to be perfectly dry ; and Dolton has given a table of such forces for 

 different tein|>uraturcs, the atmosphere being at rest, in gentle, and in 

 violent motion. In the table, the temperature 

 of the barometrical column 80 inches, and the wind blowing tnud> < 

 the value of M is 154 ; and substituting thin value in the formula, we 

 have F' = r-J. 



Th" mean mnml evaporation in (ireat Britain is 'iiono I.V1 inches or 

 01165 grains per minute: this lost number being multiplied b 

 area of a circle 6 inches in diameter, gives 0-3236 grains p. r minute 

 ( = R) from a vessel of that magnitude ; hence r'-r- 0-0647. But, by 

 Dalton's table, the elastic force (r) of vapour at a temperature equal 

 to 50* is expressed by 0-375, in inches of mercury; hence ?' 

 inches. Substituting in the above formula for u this value of 1' in 



