of a New Hygrometer. 97 



upon the tension of the pre-existing atmosphere of aqueous 

 vapour. "^ 



When the Hygrometer stands at zero ; that is, when the 

 wet and dry thermometers indicate the same temperature, 

 it may be inferred, whatever the temperature may be, that 

 the atmosphere is completely saturated with moisture ; 

 since water, at the temperature of the surrounding 

 medium, does not evaporate at all when the atmosphere is 

 completely saturated ; and the quantity by weight, a cubic 

 foot will contain, at any given temperature, may be at once 

 ascertained by reference to appropriate tables on the sub- 

 ject : but supposing the atmosphere not completely satu- 

 rated, an increase of temperature above the point of satura- 

 tion will allow a certain addition of aqueous vapour ; and 

 evaporation will commence and continue in proportion as 

 the temperature of the air is above that at which saturation 

 would take place: this process must continue either till 

 the temperature of the whole surrounding medium becomes 

 reduced to that which corresponds with the tension of the 

 pre-existing vapour, or the atmosphere becomes fully satu- 

 rated at the increased temperature, when evaporation would 

 cease, and both bulbs indicate the same temperature, show- 

 ing that the medium was fully saturated as before. 



But, supposing the atmosphere not saturated, or a certain 

 superabundant heat to be maintained, evaporation will go on 

 in proportion to the increase of temperature ; but the refri- 

 geration produced by that process will be governed by the 

 tension of the aqueous atmosphere already existing; that 

 temperature being attained, refrigeration will cease; al- 

 though evaporation continues unimpeded. 



The temperature of the moistened bulb will then remain 

 stationary : the heat necessary for converting the moisture 

 applied to the bulb into a state of vapour, being derived 

 immediately from the surrounding medium ; the tempera- 

 ture of the atmosphere regulating the quantity of water 

 raised in vapour, while the tension of the pre-existing 

 atmosphere of vapour determines the limits of refriger- 

 ation. 



2. There appears to be a diversity of opinion with respect 

 to the source of the heat which is expended in converting a 

 portion of moisture into vapour. 



VOL. IV. H 



