WATER IN THE ATMOSPHERE. 



211 



into a chamber C of which the top is closed by a black glass or silver 

 plate P. Immediately under the plate is the bulb of a thermometer th 

 round which the water flows. The water trickles out at O. As it passes 

 the plate P it cools the bulb and plate approximately at the same rate, 

 and at the dew-point P is clouded over. The tap T is then turned off 

 till the dew disappears. The deposition and disappearance are observed 

 several times, just as with the Regnault hygrometer, 

 until the two temperatures are sufficiently near together, 

 when the mean is taken as the dew-point. 



Wet and Dry Bulb Hygrometer. This consists of 



two vertical thermometers arranged side by side on a 

 frame with the bulbs projecting below the frame (Fig. 

 124). One bulb d is freely exposed to the air, while the 

 other iv is covered with muslin or wick which dips down 

 into a cistern of water just below the bulb. The water 

 rising up the threads keeps the bulb always moist. The 

 hygrometer is hung up so that the air can always flow 

 past it. If the air is saturated it takes up no water from 

 the wet bulb, and the wet a,nd dry bulbs show the same 

 temperature. If the air is not saturated then evapora- 

 tion goes on from w, and the latent heat of evaporation 

 is abstracted from the air, which cools and so keeps the 

 temperature of the wet bulb below that of the dry bulb. 

 We can see, as follows, without attempting an exact 

 investigation, how the instrument will work. Let t be 

 the temperature of the dry bulb thermometer, i.e. the 

 temperature of the air arriving, let t' be the temperature 

 of the wet bulb, i.e. the temperature to which the air 

 is cooled by the latent heat taken up. Let F be the 

 pressure of the water- vapour present in the air, / the 

 saturation pressure at the temperature of the wet 

 bulb. If the air passing the wet bulb becomes saturated, FlGfi 124. Wet 

 the pressure of the vapour is raised from F to / and and Dry Bulb 

 the latent heat taken is proportional to this rise, or is Hygrometer, 

 equal say to A (/-F). But the air is cooled from 

 t to t' giving up heat proportional to the fall or equal say to B (t t') 

 Equating the two quantities of heat 



or 



F = f-^(t- 



when the wet bulb is 



Apjohn gave for - the value 



A u\j 



above 32F. and the value -01042^-^ when it is below 32F.,p being the 



barometric height in inches and F and/ being in inches. Various 

 modifications of the formula have been proposed with the aim of making 

 it more in accordance with facts. But at the best the instrument is not 

 very exact, its indications varying with its situation and its conse- 

 quent exposure to wind, and it is probably best to use a simple formula, 



