30 Dr, Masons Description 



liable io fluctuations with every change of place and every 

 breath of wind." In the last place, I may add, in order to 

 prevent any misconception of my views : 1st. that the 

 moistened bulb will cool down to a certain point dependent 

 upon the dryness of the atmosphere, and there its tempe- 

 rature will remain stationary. 2nd. The rapidity of a 

 current will hasten the term of equilibrium ; but the degree 

 of cold induced will be found still the same. 



I will now endeavour to prove by direct experiment, the 

 law which I wish to establish in answer to this first objec- 

 tion, viz., that evaporation takes place from a humid sur- 

 face in direct proportion to the temperature and velocity 

 of the air, diminished by the force of vapour already exist- 

 ing in the atmosphere; and that under given circumstances, 

 the depression of temperature induced by evaporation must 

 have a certain limit beyond which it cannot pass ; and that 

 this depression of temperature, does not bear the same pro- 

 portion to the rapidity with which a humid surface becomes 

 perfectly dry. 



In order to prove, that, under given circumstances, the 

 depression of temperature induced by evaporation has a 

 certain limit beyond which it cannot pass ; I placed two 

 hygrometers of similar construction upon a table in the 

 middle of a large room ; they each indicated three degrees of 

 dryness. 



The one I subjected to the strongest current I could pro- 

 duce by a large pair of double bellows, which had previ- 

 ously acquired the temperature of the apartment, and found 

 by repeated trials that I could only reduce the temperature 

 0*5 of a degree below the other. The next thing to be proved 

 was, whether this depression bore a proportionate progres- 

 sive increase by equal increments of dryness. To establish 

 this fact, I waited for an opportunity to repeat the experi- 

 ment when the hygrometer indicated six degrees of dryness. 

 The results of several trials were, that the temperature 

 was depressed, just 1 degree beyond which it would not 

 pass; at 9 degrees it was depressed 1*5; after repeated 

 trials I found this invariably the case; being convinced of 

 the facts, I made the following table from to 26 degrees, 

 the greatest depression of temperature I had ever witnessed 

 in a strong Leste, during my residence in Madeira ; in 



