of a New Hygrometer. 99 



creasing until the moistened bulb has become reduced 

 in temperature corresponding to a tension of 0'670, 

 equal to a temperature of 67*5, the mean between 70° 

 and 65", when the further reduction of temperature will be 

 arrested ; the temperature remaining stationary so long as the 

 tension of the pre-existing vapour remains the same. The 

 quantity evaporated will be greater at the commencement 

 of this process ; caloric being derived from two sources, 

 namely, from the bulb itself, but chiefly from the surround- 

 ing medium ; so that the final quantity of water given off 

 will be regulated according to the temperature of the air ; 

 and will be constant and uniform. As soon as the maxi- 

 mum degree of cold has been once attained, the film of 

 water may be conceived to act as a perfect non-conductor 

 between the air and the bulb of the thermometer, for in- 

 stead of transmitting the heat which it receives from the 

 air to the thermometric fluid, it is expended in converting 

 part of that film into vapour, thus, by becoming latent, it 

 does not affect the thermometric fluid, because it is neces- 

 sarily dissipated along with the vapour into the surround- 

 ing atmosphere. 



3. When the instrument is in the shade, the tempei^ature of 

 the moistened bulb, when reduced to its utmost limits of refri- 

 geration by means of a strong current of air, indicates the exact 

 mean between the temperature of the air in the shade and the 

 Dew-point*. 



The principle which regulates this indication is the fol- 

 lowing : the urged current of air coming in contract with 

 the moistened bulb, becomes gradually reduced in tempe- 

 rature; its heat becoming latent in the process of evapora- 



* The Dew-point Hygrometer, which I have been in the habit of using, is de- 

 scribed in Jameson's Journal, July, 1835, by A. Connel, Esq. modified by myself, 

 and made by Mr. William McDowall, Infinnary Street, Edinburgh. It ap- 

 proaches most to Le Roy's method of taking the Dew-point. The instrument is 

 delicate, and when aided by a high magnifying power, the slightest deposition of 

 dew may be observed ; but it is even more difficult to use, and requires more time 

 than the beautiful instrument of Professor Daniell, the cold being produced by a 

 mixture of dried muriate of ammonia and nitrate of potash, and tlie proper addi- 

 tion of water. The chief advantage of the instrument is, that tlie whole bulb of 

 the thermometer is exposed to the refrigerating medium ; the temperature can also 

 be kept stationary for a great length of time ; and the deposit of dew is more easily 

 observed on a polished metallic surface than on glass. J. A. M. 



H 2 



