876 HYGROMETER 



some of the vapour to be condensed. The temperature of saturated vapour is said to 

 be at the dew-point. Thus, aqueous vapour of 60 Fahr. and of an elastic force of 

 518 inches is saturated, and 60 is its dew-point. These remarks apply to aqueous 

 vapour mixed with air ; differing only in vapour spreading less rapidly in a space 

 filled with air than in a vacuous space. Air is said to be saturated with moisture 

 when the aqueous vapour it contains is of maximum elastic force, the temperature of 

 the air is then called its dew-point. 



Daniell's hygrometer was the first accurate instrument invented of the kind : it 

 consists of a small glass siphon, at each end of which is a small glass bulb about 

 ] inch in diameter, a small but delicate thermometer is placed in the longer leg, as 

 much ether is poured into the lower bulb, which is of black glass, as will half cover 

 the enclosed thermometer bulb ; before sealing the tube the ether is boiled to drive 

 out all the air ; the instrument is completed by covering the upper bulb with muslin. 

 It is placed on a small brass stand, on which is placed another thermometer, whose 

 readings show the air-temperatnre, at the time the siphon shows the dew-point. To 

 make an observation, all the ether in the tube is collected into the lower bulb, and 

 from a tube or bottle a few drops of ether are poured on the muslin of the upper 

 bulb, its evaporation condenses the vapour within, fresh vapour flows along the bent 

 tube from the lower bulb, gradually the temperature is reduced, until the dew-point 

 of the surrounding air is reached, immediately a film of condensed vapour from the 

 air surrounds the black glass bulb like a ring, at the level of the ether within it, and 

 if the thermometer be read directly, almost the true temperature of the dew-point can 

 be obtained, while the exterior thermometer will be that of the surrounding air. Tho 

 expenditure of ether is an objection to this instrument, while its portability is a great 

 recommendation. 



Subsequently, several other hygrometers were devised, but none that can com- 

 pare with Regnault's Condenser Hygrometer, which is a perfect instrument of its 

 kind. It consists of a very thin and brightly-polished silver thimble, 45 mm. high 

 and 20 mm. in diameter, fitted to a glass tube, which is again fixed by a small 

 lateral tube, and a metallic tube, to a stand ; the upper end of the glass tube is closed 

 by a cork bearing a thin glass tube descending nearly to the bottom of the silver 

 thimble, and a very delicate thermometer, the pear-shaped bulb of which is in the 

 centre of the thimble ; an exactly similar thermometer, similarly placed within a glass 

 tube and silver thimble, is mounted on the other side of the stand, but the cork is 

 not penetrated by any glass tube. To mke an observation, as much good ether as 

 will more than a third fill the thimble is poured into the lesser tube of those first 

 described ; the cork is then fitted air-tight and the instrument is put in connection, 

 by means of a caoutchouc tube, with a common aspirator. Water flowing from the 

 aspirator, air is drawn through the tube, and bubbling tip through the ether, causes 

 a sufficiently rapid evaporation. After a minute or two dew will be formed on the 

 polished surface of the thimble, the thermometer should be instantly read, and the 

 stream from the aspirator stopped, another reading should be taken on the instant 

 of the evaporation of the dew ; within a few minutes the process of collecting and 

 evaporating the dew may be repeated many times, and the mean of repeated readings 

 will give a very exact point. The other thermometer will give the surrounding tem- 

 perature. The costliness of this instrument has prevented the general use it deserves. 

 Instrument-makers have rendered it less costly by omitting the second thermometer 

 and its appendages, and substituting a tube of black glass for the silver thimble. For 

 ordinary use a common test-tube, with about l inch depth of ether, in which is 

 inserted a delicate tube-thermometer, and a glass tube may be used successfully by 

 blowing through the tube, which should be of considerable length to reduce the 

 temperature. Professor Connell has invented an hygrometer, where an exhausting 

 syringe is used to lower the temperature. 



A more important instrument than either of the above mentioned is the psychro- 

 meter, or wet and dry bulb hygrometer. While with the others the constant repetition 

 of an experiment is necessary, this reduces it simply to the reading of an observa- 

 tion. It consists of two delicate thermometers, one recording the ordinary tempera- 

 ture of the air, the bulb of the other is covered with muslin, round the neck a lamp- 

 wick is twisted, conducting to a cistern of rain or distilled water, the best form of 

 which is a short small-necked glass bottle. Tho thermometers should be exactly 

 similar, and placed at about from 2 to 4 inches apart, and parallel with each other, 

 mounted on metal scales. 



When the air is completely saturated, or in that condition when it will take up no 

 more moisture, the readings of the two will l>r> the same, but as tlm atmosphere 

 becomes drier, evaporation becomes more rapid and the mercury in the wet lmH> 

 will fall more or less degrees below the ordinary temperature. Evaporation will 

 proceed from the wet bulb even below the freezing point, but the readings should 



