Daniel&rsquos Hygrometer

This instrument was invented about the year 1820, by Professor Daniel, the distinguished author of Meteorological Essays; and it entirely superseded all hygrometers depending upon the absorption of moisture. The form of the instrument is shown in fig. 77.

It consists of a glass tube, about one-eighth of an inch in diameter of bore, bent twice at right angles, and terminate

, at each end, in a bulb about one inch and a quarter in diameter. In one limb of the tube is enclosed a delicate thermometer, which descends to the centre of the adjoining bulb, which is about three-parts filled with sulphuric ether. All the other parts of the tube are carefully freed from air, so that they are occupied by the vapour of the ether. This bulb is generally made of black glass; the other is transparent, but covered with a piece of fine muslin. The support for the tube has a thermometer attached, which shows the temperature of the external air. The tube can be removed from the stand, and the parts are made to pack, with a necessary phial of ether, in a small box, which can easily be got into the pocket.

How to use the Hygrometer.—This instrument gives the dew-point by direct observation, which must be made in the following manner:—Having fixed the tube upon the stand, with the bulbs vertically downward, the ether is all caused to flow into the lower ball by inclining the tube. The temperature of the air is noted by the exposed thermometer. Then some ether is poured, from a dropping tube fitting into the neck of the phial, upon the muslin-covered bulb. The rapid evaporation of this ether cools the bulb and causes condensation of the ethereal vapour in its interior. This gives rise to rapid evaporation of the ether in the lower bulb, whereby its temperature is greatly reduced. The air in the vicinity is deprived of its warmth by the cold bulb, and is soon cooled to the temperature at which it is perfectly saturated with the vapour which it contains. Cooled ever so little below this temperature, some aqueous vapour will be condensed, and will form a dew upon the black-glass bulb. At the first indication of the deposit of dew the reading of the internal thermometer is taken: which is the dew-point.

This hygrometer has undeniable disadvantages. The surface upon which the dew condenses is small, and requires a peculiar direction of light in which to see it well. The observer, having his attention on the bulb and the thermometer, cannot always fix with precision the dew-point; and hence he is recommended to note the temperature at the appearance and at the disappearance of the dew, in order that the chance of error may be diminished. Without doubt, the necessarily long continuance of the observer near the instrument influences, to some extent, the observed temperatures; and the difficulty of not being always able to procure pure ether for the experiments is not the least of the drawbacks to the use of the instrument. Some of these disadvantages are obviated in Regnault’s hygrometer.