Page:The American Cyclopædia (1879) Volume IX.djvu/140

 132 HYGROMETRY in the air, and its form was proportionally af- fected thereby, this change could be readily indicated upon a dial, the extreme points of which are determined, the one by the least length produced by the greatest dryness, and the other by the greatest elongation caused by the most humid air that could be produced, the intermediate space being divided into 100 or other convenient number of degrees. Such an instrument would be a perfect hygrometer ; but no such substance is known, and the properties of the same body in this respect are not con- stant at all times. The best instrument of this sort, which is after all only a hygroscope, was contrived by De Saussure. It is a human hair, cleansed by boiling in alkaline water. The zero point of the scale to which it is attached is fixed by drying the hair in air rendered by chemical absorbents as dry as possible; and then, by exposing it in a receiver to air satu- rated with moisture, the other extreme of the scale is found. The equal divisions between these are assumed to indicate proportional de- grees of moisture or dryness. One end of the hair is fixed, and to the other is suspended a small weight. A grooved wheel or pulley car- rying an index is placed so as to be moved by the hair as it contracts or expands. Various other hygrometers of this class have been de- vised, some on the principle of determining the moisture by the increased weight imparted to bodies by its absorption, and others by the torsion thereby induced in cords and in vege- table fibres ; but all these methods have proved very imperfect. Two other methods are to be noticed by which the humidity of the air is ascertained. The first depends on the deter- mination of the dew point, or the degree of temperature to which the air must be reduced that its moisture shall begin to separate and condense upon cold surfaces. This difference alone is sometimes used to express the dryness of the air, as affording an indication of how near it is to its point of saturation. In tem- perate regions this sometimes amounts to 80 ; but in a dry and hot climate, under the lee of cold mountains which first strip the air of its moisture, it amounts to 60 or more ; such is the case upon the hot plains of the Deccan, to which the air is brought from the other side of the Ghauts. Cooled down upon these to a low temperature, its moisture is precipitated in rain and snow, and when immediately after this it is raised to a temperature of 90, it is found that no deposition of moisture again takes place until the temperature is reduced to 29. The observation, however, is used to furnish more exact results. Tables have been prepared with the utmost care which give the elastic force of aqueous vapor at different degrees and even tenths of degrees of temperature, ex- pressed in the height of a column of mercury sustained by the vapor. The temperature of the dew point of the air being ascertained, the elastic force corresponding to this temperature in the table represents the absolute humidity of the air, and may be converted into the ac- tual weight of moisture to the cubic foot under a given barometric pressure by the formulas prepared for this purpose, or directly by the tables constructed to reduce the labor of the calculation. By comparing the elastic force obtained from the table with that correspond- ing to the temperature of the air itself, the ratio between the two expresses the relative humidity of the air. This also is ascertained at sight by the tables specially constructed for this object. The most highly approved hygro- metrical tables are those derived from the ex- periments of Regnault, made by direction of the French government to determine the ex- pansive force of steam at different tempera- tures, which is also that of the vapor suspended in the air at the same temperatures. These tables are published in Regnault's fitudes sur I'hygrometrie, in the Annales de chimie et de physique (1845) ; and formulas also are given from which other tables, besides that of the elastic forces, have been prepared by others. The most complete series of these is furnished in the volume of " Tables, Meteorological and Physical," prepared for the Smithsonian insti- tution by Arnold Guyot, and published in the " Smithsonian Miscellaneous Collections," 1858. In the same series is also 'presented the table of elastic forces of vapor deduced from the ex- periments of Dalton, together with others bnsed upon it, and in general use in England. These are also found in Glaisher's " Hygrometricnl Tables " (London, 1847), and in the " Green- wich Observations." Various forms of the dew-point instrument or hygrometer have been devised. That of Prof. Daniell, which has been much used, is of the following construction : A bent tube, blown out at each end to a bulb, is laid across the top of a pillar, which serves as a stand, the two bulbs hanging down one on each side. One arm of the tube is long enough to contain a delicate thermometer, the bulb of which terminates in some ether contained in the external- bulb. By boiling the ether be- fore closing the tube the air is nearly ex- pelled. When in use the empty bulb is cov- ered with a piece of muslin, which is kept wet with ether. The evaporation of this con- denses the vapor within, causing the liquid in the other bulb to evaporate and grow cool. The bulb becomes at last sufficiently cool for the moisture to condense upon it, and the in- stant this makes its appearance in the form of a ring of dew encircling the bulb at the level of the surface of the ether, the temper- ature is to be noted by the thermometer with- in, while that of the air is observed upon another thermometer attached to the stand. Another observation of the enclosed thermom- eter is made as the dew disappears by the bulb returning to its former temperature ; and the mean of the two observations will give a close approximation to the dew point. A better in- strument is that of Regnault. Two glass tubes are suspended by a small tubular arm near the