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.
(Fig. 78) consists of a tube, _C_, made of silver, very thin, and perfectly polished; the tube is larger at one end than the other, the large part being 1·8 inches in depth, by 0·8 in diameter; this is fitted tightly to a brass stand, _B_, with a telescopic arrangement for adjusting when making an observation.
The tube, _C_, has a small lateral tubulure, to which is attached an India-rubber tube, with ivory mouth-piece; this tubulure enters _C_ at right angles near the top, and traverses it to the bottom of the largest part.
A delicate thermometer, _D_, is inserted through a cork, or India-rubber washer, at the open end of the tube, _C_, the bulb of which descends to the centre of its largest part.
_G_ is an attached thermometer for taking the temperature of the air, and _F_ is a bottle containing ether.
_To use the Condenser Hygrometer_, a sufficient quantity of ether is poured into the silver tube to cover the thermometer bulb: on allowing air to pass bubble by bubble through the ether, by breathing in the tube, _E_, an uniform temperature will be obtained; if the ether continues to be agitated, by breathing briskly through the tube a rapid reduction of temperature will be the result; at the moment the ether is cooled down to the dew-point temperature, the external surface of that portion of the silver tube containing ether will become covered with a coating of moisture, and the degree shown by the thermometer at that instant will be the temperature of the dew-point.