82 On the Hygrometer hij Evaporatiov. 



2. We may suppose two different tliennometers ; one for 

 mark'ng the general temperature ; and the other, having its 

 bulb dipped in water, or covered with a wet Hnen rag, or with 

 bibulous paper soaked in water, for the purpose of ascertain- 

 ing tlie cold produced by the evaporating power of the air. 

 Now, if we suppose that the atmosphere is saturated with 

 moisture, its contact with the wet bulb will produce neither 

 any evaporation from the humid surface, nor any deposition 

 of moisture upon it from the air. As there is no action be- 

 tween the air and the bulb, neither heat nor cold will be pro- 

 duced ; and the mercury will stand at the same height in both 

 thermometers. But if the air be not saturated with moisture, 

 there will be an evaporation from the wet bulb ; and, evapora- 

 tion being invariably attended with the disappearance of heat 

 of temperature which enters into the constitution of the va- 

 pour in a latent ibrm, the mercury of the thermometer will be 

 depressed. The air in contact with the moist surface will 

 therefore become damper both by the depression of its tem- 

 perature and by the small adtlition of humidity diffused in it. 

 If, in a single instant of time, the point of saturation with 

 humidity be not fully attained, the same process will be re- 

 peated in a second and a third instant, until at length the ail' 

 is charged with all the vapour it is capable of retaining at its 

 depressed tem])erature. 



It is certain that the thermometer cannot be depressed be- 

 low the point at which the air is saturated with humidity. 

 Even ii', by any accidental cause, we sup})ose a greater degree 

 of cold to be proiluced, the consequence would be a deposi- 

 tion of part ol' the vapour in the licjuid form, attended with 

 an evolution of heat; by which means, supposing the extra- 

 neous cooling cause to be removed, the thermometer would 

 again rise and settle at the point of saturation. 



In perfectly still air it is known that evaporation goes on 

 very slowly; and this circumstance may make it difficult to 

 discern when the action between the air and the bulb has 

 come to an invariable state. Some agitation of the instru- 

 ment, or some motion in the air, seems to be proper, whether 

 to hasten the permanent degree of cold, or to ascertain that 

 it h.as actually taken place. 



On the other hand, every portion of air must take up some 

 definite time in parting with its heat and absorbing its share 

 of vapour; and if we su])pose a circulation so brisk as to dis- 

 place the air from the thermometer in half that time, it is 

 plain that it would be only half saturated with moisture by its 

 contact with the wet bulb ; in which case the experiment would 

 be unsuccessful. This however is only an extreme case, which 



it 



