Ether is poured into the silver tube in sufficient quantity to cover the bulb of the thermometer. The ether is then agitated by breathing through the flexible tube until, by the rapid evaporation thus produced, a condensation of moisture takes place, readily observable on the bright polished silver surface, and the temperature indicated by the thermometer at that moment is the dew-point.
39.
Regnault’s Hygrometer. Scale
about 1/10.
Daniell’s Hygrometer, or Dew-point Thermometer (Fig. 40), consists of a glass tube, bent twice at right angles, each extremity terminating in a bulb about 1-1/2 inch in diameter, supported on a brass stand, to which a thermometer is attached to indicate the temperature of the surrounding air. The lower bulb is of blackened glass, to facilitate the observation of the dew-point; it is about three parts filled with pure ether, and contains a very delicate thermometer. The upper bulb at the extremity of the short stem is transparent, but covered with thin muslin, upon which, when an observation is made, pure ether is slowly dropped. The evaporation rapidly lowers the temperature, until a moment arrives at which dew condenses on the black bulb. A quick eye is necessary to note this and the temperature shown by the thermometer simultaneously, the latter showing the degree at which the atmosphere is saturated with moisture at the time of observation. To avoid error, it is usual to note the temperature at which the dew disappears, and take the mean of the two temperatures.
40.
Daniell’s Hygrometer.
Scale about 1/5.
Dyne’s Hygrometer, for showing the dew-point by direct observation, by means of iced water and black glass, enables the observer to dispense with the use of ether, and shows the dew-point with great distinctness.
41.
Mason’s Hygrometer.
Scale about 1/6.
The hygrometer in most general use is the wet and dry bulb thermometer, and for which Mr. Glaisher has calculated an elaborate set of tables, a brief abstract of which sufficient for general purposes is subjoined.