35. There are many substances which absorb moisture from the atmosphere and which swell in volume in consequence. Many attempts have been made to utilize this property, to construct instruments which would indicate the density of the moisture in the air. Until recently not any have been produced which were accurate or reliable. The most successful instrument of this kind is shown in Fig. 1. The indicator hand is attached to a metallic spiral, which is filled with absorbent material. This material absorbs moisture from the air in proportion to the density of the atmospheric vapor, and gives it off again as the density decreases. The material swells in proportion to the amount of moisture which it holds, and thus the spiral which contains it is compelled to bend.

Measurement Of Humidity 109

Fig. l.

36. The humidity of the atmosphere may be measured by means of two thermometers, one of which is perfectly dry, and the other provided with a wet cloth over its bulb. The cloth is kept moist by a thread or wick leading from a small tank. If the air is not saturated, the water will evaporate from the cloth and thus cool the bulb. The rapidity of the evaporation depends upon the relative dryness of the air, and the depression of the thermometer indicates approximately the rate of evaporation from the cloth. This device is called the wet-and-dry-bulb thermometer. The scale of humidity is arbitrary, and the operation of the instrument is not sufficiently accurate to be satisfactory.

37. The humidity of the atmosphere may also be measured with accuracy by observing the temperature at which the vapor, or atmospheric steam, will condense. Having found the temperature at which condensation takes place, the pressure of the steam per square inch, and its weight per cubic foot, may be found from Table 8 following. The weight of moisture which is said to be absorbed by a cubic foot of air is the weight of a cubic foot of steam at the pressure and temperature thus found.

The instruments commonly used for this purpose are called dew-point hygrometers. They operate by gradually cooling a bulb or plate of black glass below the atmospheric temperature. As the temperature falls, a point will be reached at which a deposit of dew begins to appear upon the glass. The formation of dew signifies that the atmospheric vapor which is actually present condenses at the temperature which the glass then possesses. Black glass is preferred, because it readily shows the slightest film of dew.

38. Daniell's hygrometer is shown in Fig. 2. It consists of a glass tube A having a bulb at each end, as shown. The bulb B is made of black glass, and is partly filled with sulphuric ether. The bulb D is empty, and is covered with muslin. A small mercurial thermometer C is enclosed in the tube A, and is so suspended that its bulb extends below the surface of the liquid ether. This thermometer indicates the temperature of the liquid in the bulb B. A second thermometer E, which is attached to the wooden stand, serves to indicate the temperature of the surrounding air. To operate the instrument and obtain a deposit of dew upon the bulb B, a small quantity of ether is poured over the muslin on bulb D. The ether evaporates rapidly, and thus absorbs heat from the bulb and the ether vapor contained in the instrument. As the temperature falls, a point will be reached at which a deposit of dew will appear upon the surface of the bulb B. The dew will remain there until the ether wholly evaporates from the muslin covering of D and the instrument begins to recover its normal temperature.

If too much ether is poured upon the cloth-covered bulb D, the instrument may be cooled to such an extent as to depress the thermometer E and cause it to give a false indication of the temperature of the atmosphere.

The temperature shown by the thermometer C at the exact moments when the dew appears and disappears must be carefully noted. If there is much difference in the first 4-13

Measurement Of Humidity 110

Fig. 2.

and the second reading, the test should be repeated. When the test is properly made, the average of the two readings may be taken as the correct dew point.

The instrument should be placed in a position where it will be free from the influence of any heating agency. The operator should take care that the warmth of his hands or breath does not affect the thermometers.

39. Having found the dew point, the following rule may be used to compute the relative humidity.