Mr Foggo 071 the Dexv-Poiiit Hygrordefir. 41 



Philosophical Journal, (vol. xxiv. and xxv.) before it was 

 known that a similar method had been adopted by Mr Jones, 

 and long before we had Jearnt from Mr DanielPs remarks 

 upon the instrument of this last gentleman that it had like- 

 wise been used in Germany. Mr Daniell has observed, that, 

 as the bulb is only partially exposed to the cooling effect of 

 the ether, we cannot thus obtain the true term of deposi- 

 tion, since the thermometer is graduated on the supposition 

 that the whole of the mercury is of the same temperature. 

 In short, he wishes us to believe that the two halves of the 

 thermometer have different temperatures. It would be easy 

 to oppose these theoretical objections by others more sound, if 

 it were not absurd to try a question of this kind by any thing 

 but experience ; and a description of the manner of experiment- 

 ing will enable every one to judge for himself on the subject in 

 dispute. I use a thermometer with a ball of an oval shape, blown 

 of black enamel ; three-fourths of its surface are covered with 

 muslin ; a ring of silver or brass separates the covered part from 

 that on which the dew is deposited. Haviii^ taken the tem- 

 perature of the air, ether is dropped slowly on the muslin, the 

 evaporation from which cools the mercury slowly and equably 

 at the rate of one degree in T' or 8''. Hence, the contraction of 

 the mercury is so very gradual that every portion of it has time 

 to acquire the same temperature. If it be thought it may 

 be otherwise, the instrument itself can detect the possible 

 error ; for, after the dew-point has been thus taken, and its 

 approximate temperature known, the dew may be wiped off, 

 and by cautious management the heat of the bulb may be 

 kept steadily for any length of time a little above the exact 

 point necessary to precipitate the moisture. Then, all the 

 mercury having had full time to acquire the same degree of 

 heat, when it is allowed to fall, and the dew-point again 

 taken, if there has been any error in the first observation it 

 will now be apparent. I have often verified my observations 

 in this manner, but I always found it unnecessary. In Mr 

 DanielPs hygrometer the evaporation of the ether in the sefl- 

 tient ball is extremely rapid, sometimes like an explosion, and 

 in damp weather, when the dew-point differs little from the 

 temperature of the air, the deposition is instantaneous. Be- 



