[Illustration: FIG. 51. STEVENSON’S THERMOMETER SCREEN.] The wet bulb is covered with a single layer of soft muslin, while a noose of six to eight strands of darning cotton connects the neck of the wet bulb with a covered water receptacle 2 to 3 inches distant, below and at its side. This receptacle is kept filled with rain-water. [Illustration: FIG. 52. SYMONS’ EARTH THERMOMETER.] [Illustration: FIG. 53. DRY AND WET BULB THERMOMETERS.] From the readings of the dry and wet bulb thermometers three deductions can be made: 1. The temperature of the dew point. 2. The elastic force of aqueous vapour. 3. The relative humidity. The =dew point temperature= is that temperature at which the outside air at the time the observation is taken will deposit the moisture contained in it. It is the temperature at which the air is saturated with moisture. It is calculated from the readings of the wet and dry bulb thermometers (_a_) by Glaisher’s tables; (_b_) by Apjohn’s formula. Glaisher’s tables are based on a series of numbers called Greenwich or Glaisher’s factors, which he determined by comparison between observations made with the dry and wet bulb thermometers and with Daniell’s hygrometer. The formula for using the factors is as follows:— d = D - {(D - W) × f} where d = dew point, D = dry bulb temperatures, W = wet bulb temperature, and f = factor. The following examples are from Glaisher’s table of factors. ┌───────────────────┬───────┐ │READING OF DRY BULB│FACTOR.│ │THERMOMETER FAHR. │ │ ├───────────────────┼───────┤ │ 55° │ 1·96 │ │ 56° │ 1·94 │ │ 57° │ 1·92 │ │ 58° │ 1·90 │ │ 59° │ 1·89 │ │ 60° │ 1·88 │ └───────────────────┴───────┘ Thus, if D = 60°, W = 55°, then dew point = 60 - {(60 - 55) 1·96} = 50°·2. The dew-point may also be obtained by Apjohn’s formula; which for a pressure of about 30 inches is F = f-(D-W)/87 D being dry and W wet bulb temperature, F elastic force of vapour corresponding to dew-point, and f, elastic force corresponding to wet bulb temperature (ascertained from a table of tensions). The _elastic force of aqueous vapour_, _i.e._ the amount of barometric pressure due to the vapour present in the air is dependent upon the temperature of the dew-point. It is given for every tenth of a degree of temperature in Table VI. (p. 42) of Marriott’s _Hints_. The _relative humidity_ is a term expressing the percentage of saturation of the air with water vapour. It is obtained from Table VI. (above) as follows:— Elastic force of water vapour at the temperature of the Relative } dew-point } = —————————————————————————————————————————————————————————— Humidity } Elastic force of water vapour at the temperature of the air (_i.e._ the dry-bulb reading.)