of a New Hygrometer. 29 



ing circumstances occurred : either the temperature of the 

 medium must increase so as to render it capable of receiving 

 a further supply of vapour, which would be at once shown 

 by the depression of the mercury in the moistened bulb, or 

 the air must be rendered less humid by a sudden depression 

 of temperature, or increased density of the atmosphere, 

 which would condense the vapour already existing in the 

 medium, and cause precipitation in the form of rain or dew, 

 when the temperature would be again raised to a certain 

 degree by the heat given out from the water, passing from 

 a state of vapour to its fluid condition, it would then admit 

 of a further portion of vapour, and the mercury in the 

 moistened bulb would descend as before. 



Under either of the above conditions, were the air in 

 motion, the rapidity of evaporation and consequent depres- 

 sion of temperature would be found exactly equal to the 

 velocity of the current, diminished by the force of vapour 

 already existing in the atmosphere. 



From the above remarks it will be seen that the ^^ celerity 

 of evaporation has been mistaken for its intensity, and the 

 coldness induced on the evaporating surface has been viewed 

 as the accumulated effect of a rapid dissipation of moisture ; 

 whereas the fact is simply, that the quantity of particles 

 will be carried away in proportion to the velocity of the 

 wind : consequently a humid surface will be much more 

 rapidly dried ; but it does not follow as a consequence of the 

 rapid dissipation of moisture that the temperature of the 

 evaporating surface should be proportionally depressed, for 

 in a free atmosphere, as Sir J. Leslie has proved, vaporiza- 

 tion proceeds with unabated energy, while the correspond- 

 ing depression of temperature must advance by a relaxing 

 progression ; since otherwise, the accession of an accelerated 

 movement might push it to any extent, but the reduced 

 temperature caused by this process under given circum- 

 stances has a certain limit beyond which it cannot pass."* 



The chief objection; therefore, against this instrument, 

 advanced by Mr. Daniell himself, is perfectly without foun- 

 dation, viz., " that the temperature of evaporation is no 

 longer that constant quantity which it is supposed to be if 

 dependant only upon the temperature of the air, and is 

 * Leslie on Heat. 



