25. Mr Leslie has proposed an hygrometer, totally dif ferent in principle from any of those we have considered, but perhaps superior to all of them, both in point of ac curacy and delicacy. Hygrometers formed of organic substances are liable to be affected by the partial decom positions, 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 wea ther better than others, none of them can he said to be in destructible, arid all of them, in the course of time, lose in a great degree their hygroscopic properties. Their scales, therefore, how ever accurately constructed at first, are sub ject to a gradual derangement, and require occasional ad justments to render their indications at all correct. This is certainly a great objection to the use of these instru ments ; 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 con tained in a given volume of air may be accurately deter mined, in terms of the degrees of its scale, we must now consider it as by far the most accurate hygrometer that has yet been proposed. The insti ument consists of two spheres of glass A, B, Fig. 8. connected with each other by a bent tube CDEF, which is fixed to the stand GH, and contains inclosed a small portion of sulphuric acid, tinged with carmine to render it more distinctly visible. When the spheres, both of which are filled with air, arc at the same temperature, the liquor in the recur red tube re mains stational y ; but if the balls, as A, be colder than the other B, the air in the latter, by its greater elas ticity, immediately depresses the liquor in the limb FE, and raises it in an equal degree in the limb Cll. One of the balls is accordingly covered with a coating of cambric, or tissue paper, and kept continually moist with pure wa ter, conveyed to it by filaments of floss silk from an ad joining 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 de pression of the liquor in the limb FE becomes an indica tion of the relative dryness of the surrounding air. The caloric abstracted from the moistened hall by evaporation is incessantly supplied by the air and the contiguous bo dies, and in the course of two minutes the maximum of effect is produced. Were it not for this continual influx of temperature, no limits could be assigned to the degree of cold that might be induced. The scale is formed by dividing the interval between the boiling and freezing points into one thousand equal parts, so that 10° correspond to 1° of the centigrade thermometer, and 50° to 9" of Fahren heit. This hygrometer acts equally well when the mois ture 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 tem perature 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 I° in 7°, to adapt the scale to the actual state of things.
26. When the instrument is intended to be portable, Mr Leslie prefers the form delineated in Fig. 9. The two balls, being in the same perpendicular line, are protected from injury by a case of wood or ivory ; and the instru ment may thus be transported from one place to another with perfect safety. We shall conclude our description •of this simple but ingenious instrument, by remarking, that an ordinary thermometer, having its bulb covered with moistened paper, gives the same indications, if its tempe rature be subtracted from the temperature of the air de termined by a naked thermometer, placed in similar cir cumstances with the other.
Relation between the Indications of Hygrometers, and the absolute Quantities of Moisture in Vapour.
27. Having thus described the construction of the vari ous kinds of hygrometers which have hitherto been em ployed, we shall now proceed to investigate the relation subsisting between the indications of these instruments, and the absolute quantities of moisture existing in a given volume of the medium to which they are exposed. To enable us to prosecute the subject with sufficient precision, it will be necessary, in the first place, to take a concise view of the quantity of vapour contained in a vacuum, at different temperatures ; and, secondly, of the quantity of it which can exist, in the same circumstances, under dif ferent pressures, in mixture with air. The experiments of Mr Dalton on the elasticity of steam at various tempe ratures, together with the recent researches of Gay Lussac, will enable us to solve both these problems with the most perfect precision.
28. Mt Dalton has given a table, containing the results of his admirable experiments on the force of steam for every ilegree of Fahrenheit's thermometer, from zero to 325°, from which Biot has deduced the following Table, adapted to the centigrade scale.
in a geometrical series. In order to obtain a general ex pression for the law of its increase, Biot assumes that the ratio of the terms is constant, and equal to k ; then calling F „ the elastic force, corresponding to the temperature 100— n, The supposition, on which this expression is founded, though not rigidly true, will lead to results sufficiently conformable to experiment to justify us in adopting it. The quantity n, Log. k, may be exhibited by a succession of terms of the form a n b + ens + Etc. and the ex pression then becomes, _ the co-efficients a, b, c being constant, and determinable from three equations in which F n is given, and conse quently n. It is unnecessary to take more than three terms of the series, as the co-efficients of the powers of n will be found to diminish much faster than the powers them selves increase. To determine the co-efficients a, b, and c, Biot employs the elastic force of vapour for the tempera tures 25, 50 and 75, reckoned downwards from the boiling point ; thus, we have, 29. The first column of this Table contains the tempe rature, in degrees of the centigrade scale, at the interval of 6} degrees ; the second, the elastic force of vapour in English inches ; and the third, the relation in which each term of the elastic force stands to the one immediately above it. It is obvious, that if the same relation suhsisted bet%teen the terms, in successive order, the numbers in the third column would form a series of quantities in geo metrical prog"cssion, the first term of which would be .2, and the last 30. The terms, however, continually decrease in a slow and regular manner, as the temperature increases, and tnelefore the elastic force of vapour cannot proceed By help of this formula, we have calculated the following Table of the elastic force of steam, Qom zero to 100° of Fahrenheit, which includes the ordinary range of natural temperature. We have also annexed a column, exhibiting the elastic force of vapour for the same range of tempe rature, as determined by Mc Dalton. The difference be tween the corresponding pumbers in the two columns sel dom exceeds the 1000th part of an inch, except between 75° and 90°, where Mr Dalton's table seems to be a little faulty. •