H Y G R O M E T R Y. 



593 



lr^. ..' I 

 ol' :..':- 



which could exist in a cubic inch of air at the former Hygrome- 



temperature being .0025476 grains, and at the latter "? 



.0017823, the relative moisture of the air would be ex- ""V" 

 pressed by the fraction 4fr*i> or - fi 96, complete satu- 

 ration being denoted by unity ; ftl like manner, if the 

 temperature of the air were 70, and the point of depo- 

 sition 58^, the relative humidity of the air would be 

 also .6996. Now, in the state of things we have sup- 

 posed, the actual quantities of moisture in the air 

 would be to each other, as 17823 to 3-2296, or as 1 to 

 2 nearly, and yet the hygrometer of Saussure, or that of 

 De Luc, would, in both cases, stand at the same points 

 of their respective scales, the former at 8 j" and the 

 latter at 50. The quantity of moisture absorbed by 

 hygroscopic substances would also be the same, or 

 nearly the same, in the circumstances stated ; so that 

 hygrometers, though indicating merely relative differ- 

 ences, may nevertheless be employed for practical pur- 

 poses, to point out the extent to which such bodies are 

 affected in their weight by humidity, after the relation 

 subsisting between their increase of weight and the de- 

 grees of the instrument has been determined by expe- 

 riment. 



90. As the organs of living beings are relaxed by Correspo:.- 



This Table may be employed to calculate, by approxi- 

 mation, the number of grains in a cubic inch of air, 

 when the degrees of De Luc's hygrometer, the tempe- 

 rature, and barometrie.il pre.=ure are known. Thus, if 

 we employ the data of the example in 62, in which 

 De Luc's hygrometer stood at 27, the- thermometer at 

 fj5 e , and the barometer at 20.35* : The tension cor- 

 responding to 27 in the Table, is found by taking a 

 proportional part for- the excess above 25* to be .403 ; 

 and the weight of the moisture in a cubic inch of air, 

 for theentire tension of the ten.perature baring .0039589? 



grains, and corrected for barometrical pressure humidity, somewhat in the same manner as the fibres dcnce bc- 

 .003873 1 9 grains, we thus obtain .003873 1 9 x .403, or of inanimate substances, the indications of hygrometers tw n *> 

 .00 1 56089 grains, as the weigl.t of the moisture in a serve to apprize us of the effects which the moisture ^^"1"* 

 cubic inch of air in the given circumstance*. This of the atmosphere, such as it exists in that fluid, is cal- g rornetc '" 

 result coincides very nearly with the quantity of mois- ciliated to produce on the general tone of the muscu- ^A the e'f- 

 ture deduced from the same data by the formula in tar frame : and that, too, in a way much more direct fecti of 

 $ 62. and immediate, than if the graduation of their scales moisture in 



89. It must be sufficiently obvious, from all we have had referred to absolute, instead of relative degrees of ,",", 

 stated, that the degrees marked by hygrometers, of moisture. In proof of this assertion we need only re- } 

 every description, are entirely relative ; and that the mark, that the quantity of moisture in a warm apart. 



ment is generally greater than that contained, at the 

 same time, in an equal volume of the external air ; and 

 yet if we were to judge either by our feelings, or the 

 apparent indications of the hygrometer, we should be 

 led to draw an opposite conclusion. The hygrometer, 

 therefore, though it only announces relative degrees of 

 humidity, conveys to us the species of information re- 

 specting the state of the air, in which we feel most in- 

 terested . namely, the extent to which its hygroscopic 

 condition will affect our personal comfort. 



91. In this country, the mean height of De Luc's Mean itate 

 hygrometer, in the open air, is about 65". Above this of the ]\- 

 pomt. the degrees of the instrument may be considered granettr. 

 a indicating the state of the air to be damp and un- 

 wholesome. In a room heated to the temperature of 



60, the index remains pretty steadily at 35, which 

 corresponds nearly to tin- air being half saturated with 

 moisture. A thcrmometc r, with a moistened bulb, 

 stands in the same circumstances, from 9 to 10 be- 

 low the temperature of the room. Mr Leslie mentions, 

 that on one occasion his hygrometer stood at Paris in 

 the month of September, at 120, so that a thermo- 

 meter with a moistened bulb would have remained, 

 in the amc circumstances, 21.6 below the tempera- 

 ture of the air. 



92. Having given a detailed view of the nature of p roo f, t- 

 hygrometric scales, both as they refer to absolute and direct ex- 

 relative degrees of humidity in the ambient medium, periment, 

 we shall devote the remainder of this article to the ex- that l ?*^. 

 planation of some important meteorological appear- '"'V 9 

 ances more immediately connected with hygroinctry. ,,f vapoU r 

 It appears by the principles which we have established, by tempe- 

 rature onk. 



It appaan by Uw JfcSMnfcpcarf Ja<m*l of ih Royal Society, lhat the mean height of De Luc' hygrometer, at London, Is about 



During a parted of 11 /can, from 179* to 1903, the lowest height at which It was observed, ws* SO", 

 vol. xi. PAKT ii. 4 r 



absolute quantity of moisture held in the vaporous 

 state, in any medium, cannot be accurately inferred 

 from these instruments, unless the temperature and 

 barometrical pressure be also taken into account. 

 Though, in one respect, this property of indicating re- 

 lative degrees of humidity, may be considered as an 

 imperfection of hygrometers, it renders them more va- 

 luable for certain purposes, than if they pointed out, 

 in a less indirect manner, the actual quantity of mois- 

 ture in a given volume of the medium in which they 

 are placed. By expressing, for instance, the relative 

 condition of the atmosphere with regard to moisture, 

 they shew at the same time its capacity for receiving 

 an additional portion of vapour in admixture with it ; 

 or, what is of equal importance, its tendency to deposit, 

 by a diminution of temperature, part of that which it 

 already contains. Hence, for the ordinary purposes of 

 meteorolgy, they are more useful than if they indica- 

 ted absolute differences ; and as the extent to which 

 they are affected, corresponds to the effects of moisture 

 on other hygroscopic bodies, placed in similar circum- 

 stance*, they may be used with equal advantage to point 

 out the simultaneous condition with respect to humi- 

 dity of various articles of commerce, such as corn, cot- 

 ton, wool, silk, &c. the portion of water absorbed from 

 the air by these substances being regulated, not by the 

 absolute quantity of moisture existing in the atmo- 

 sphere, in a vaporous state, but by the relation which 

 the latter quantity bean to the whole moisture that 

 could be maintained in that state, at the temperature. 

 Thus, if the temperature of the air were 50, and the 

 point of deposition 40*, the whole quantity of moisture 



