Hygrometry (Gr.Hygrometry 090063 moist, andHygrometry 090064measure), the method of determining the amount of moisture in bodies, more especially in atmospheric air. A hygrometer is an instrument used for this purpose; and a hygro-scope is any substance that absorbs moisture from the air, and is in consequence changed in form or weight. Various salts absorb moisture and deliquesce, and are consequently called hygroscopic. These serve as hygrometers in chemical analysis; thus chloride of calcium placed in a glass tube absorbs the moisture from the air passed through the tube, and its increase of weight determines the quantity. The property is exhibited in hemp and cotton ropes, and in small fibres, as those of whalebone, and in hairs. Paper by absorption of moisture expands to such a degree that it is an imperfect material for preserving accurate plans. Its variation in length in extremely dry and in moist air sometimes exceeds 1 in 40. If a substance could be found which absorbed moisture in proportion to the quantity in the air, and its form was proportionally affected 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 constant 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 saturated with moisture, the other extreme of the scale is found. The equal divisions between these are assumed to indicate proportional degrees 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 carrying 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 devised, 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 vegetable 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 determination 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 temperate regions this sometimes amounts to 30°; 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, expressed 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 actual 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 corresponding 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 experiments of Regnault, made by direction of the French government to determine the expansive force of steam at different temperatures, which is also that of the vapor suspended in the air at the same temperatures. These tables are published in Regnault's Etudes sur l'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 institution 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 experiments of Dalton, together with others based upon it, and in general use in England. These are also found in Glaisher's "Hygrometrical Tables" (London, 1847), and in the "Greenwich Observations." - Various forms of the dew-point instrument or hygrometer have been devised. That of Prof. Paniell, 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 before closing the tube the air is nearly expelled. When in use the empty bulb is covered with a piece of muslin, which is kept wet with ether. The evaporation of this condenses 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 instant 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 temperature is to be noted by the thermometer within, while that of the air is observed upon another thermometer attached to the stand. Another observation of the enclosed thermometer 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 instrument is that of Regnault. Two glass tubes are suspended by a small tubular arm near the top of each, both opening into the hollow stand that supports the tubes. A pipe for exhausting the air by means of a sort of bellows or the flow of water connects with the hollow in the stand by an opening near its base. The two tubes are closed, each with a cork through which a thermometer tube is fitted, the bulb in one reaching nearly to the bottom. Over the lower end of this one a very thin and highly polished thimble of silver nearly two inches long is fitted, and a fine tube open at each end is passed through the cork, reaching from the external air nearly to the bottom of the tube.

Ether is poured into this bulb, covering the lower end of the thermometer, and rising an inch or two higher than the upper edge of the silver thimble. To determine the dew point, the apparatus for exhausting the air from the hollow stand is set in action. This causes the air to pass through the fine tube, and bubble through the ether, keeping it in motion and taking up its vapor. The liquid, the thermometer bulb, and the silver coating of the tube equally feel the reduced temperature, and the instant this reaches the dew point, the whole surface of the silver is covered with moisture. The temperature of the thermometer placed in the ether is then observed, while the other marks the temperature of the air. By stopping the current of air the temperature rises, and the moisture disappears from the silver. The thermometer is to be noted again, and the mean of the two observations taken for the dew point; or several trials may be made in rapid succession. To avoid affecting the result by the warmth radiated from the body, a small telescope maybe used in reading the thermometer.

The instrument has been modified by Prof. Connell in substituting for the tube a small flask of highly polished brass or silver, into the neck of which is secured an exhausting syringe. - The second of the two methods above referred to, by which the humidity of the air is ascertained, involves the determination of the temperature of evaporation; and the instrument used is the wet-bulb thermometer or psychrometer invented by Prof. August of Berlin, and described in his work Ueber die Fortschritte der Hygro-rnetrie (Berlin, 1830). It consists of two delicate thermometers placed near together. The bulb of one is covered with muslin, which is kept wet by water supplied from a vessel close by through capillary conduction. The instrument is placed in a light draught of air, and as evaporation goes on the mercury in the wet-bulb thermometer sinks to a certain point; the temperature of both is then noticed. If the air was nearly saturated with moisture, the difference will be found to be very slight. The barometric pressure is observed at the same time, and data are thus afforded for calculating the elastic force of aqueous vapor in the air.

The formula for this calculation, modified by Re-gnault, and the psychrometrical tables deduced from it, are given in the volume of tables referred to above, and are equally applicable to the estimation whether the dew-point instrument or wet-bulb thermometer is used. To render them more convenient, they have been converted by Prof. Guyot into English measures. The series also contains tables of the weight of vapors in a given space at different temperatures. The method by the wet bulb, though regarded as decidedly the most convenient means of determining the elastic forces of the vapor, and thence the humidity of the air, is still rendered somewhat uncertain in its results from the impossibility of keeping the wet bulb uniformly moist, and from other causes also. The uncertainty of its results is indeed in some cases so great that Regnault in 1872 recommended that, for accurate meteorological purposes, resort should be uniformly had to the chemical methods of extracting and weighing the aqueous vapor in a given volume of air.

To this end he has devised a simple arrangement by which concentrated sulphuric acid may be exposed to the atmosphere and absorb its aqueous vapor; a method that is specially applicable at very low temperatures. - The ultimate object of these hygrometrical investigations is, by enabling the meteorologist to ascertain at all times, in all localities, and at all accessible elevations, the true condition of the atmosphere as to moisture, to furnish him with accurate data for studying the laws which control its variations. The following table of relative humidity is prepared for every 5° F. from 5° to 95° above zero, and for a difference of temperature between the air and the dew point, technically called the complement of the dew point, ranging from 0° to 18°. (See Dew Point, in article Dew.)

Table Of Relative Humidity Of The Air

TEMP. OF AIR.

DIFFERENCE OF TEMPERATURE OF THE AIR AND OF THE DEW POINT.

0o

1o

3o

6o

7o

9o

10°

12°

14°

16°

18°

5°.........

100

96

91

87

S3

80

76

72

69

66

63

57

52

47

43

10.........

100

96

91

87

88

80

76

73

70

66

63

58

53

47

43

15.........

100

96

91

87

88

80

76

78

70

66

64

58

53

48

44

20.........

100

96

91

87

88

80

76

78

69

66

68

58

53

48

44

25.........

100

96

91

87

84

80

76

73

70

67

64

58

53

48

45

30.........

100

96

92

88

84

81

77

74

70

67

65

59

54

49

45

85.........

100

96

92

88

84

82

77

74

71

68

66

60

55

51

46

40.........

100

96

92

89

85

82

78

75

72

69

67

62

56

52

48

45.........

100

96

93

89

85

83

79

75

73

70

68

63

57

53

49

50.........

100

96

98

89

86

83

80

76

74

71

69

63

58

54

50

55.........

100

96

93

90

86

83

80

77

74

72

69

64

59

55

51

60.........

100

96

93

90

86

84

81

77

75

72

70

64

60

56

52

65 .....

100

97

98

90

87

84

81

78

75

72

70

65

61

56

52

7O.........

100

97

98

90

87

84

81

78

76

73

71

65

61

57

53

75.........

100

97

94

91

87

84

82

79

76

73

71

66

62

58

54

80......

100

97

94

91

88

85

82

79

77

74

72

67

62

58

54

85 ....

100

97

94

91

88

85

83

80

77

74

72

67

63

59

55

90 ......

100

97

94

91

88

85

83

80

77

75

73

68

63

59

56

95 ......

100

97

94

91

88

85

83

80

78

75

73

68

64

60

56