70 



NEGEETTf AND ZAMBKA, HOLBORN VIADUCT, E.C., 



FIG. 75. FIG. 76. FIG. 77. 



covered with a piece of fine muslin. The support for the tube has a delicate 

 thermometer attached, to show the temperature of the external air. 



This instrument gives the dew-point by direct observation, and is to be 

 used at an open window facing the north 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 ^Efcher is poured 

 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 Mtlaer 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. In very damp or windy 

 weather the .^Ether should be slowly dropped on the bulb, otherwise the descent 

 of the mercury in the Thermometer is so rapid as to render it difficult to be 

 certain of the temperature. Should this occur, the observation may be repeated 

 by watching the temperature at which the ring of dew disappears, the mean of 

 the two readings will be the correct point of precipitation. The greatest differ- 

 ence observed by Mr. Daniell in the course of four months' daily experiments 

 between the external thermometer and the internal one at the moment of 



