On a Portable Hygrometer. 351 



Art. XII. — On a Portable Hygrometer ; by A. A. Hayes. 



By presenting the meteorologist and chemist, with his beautiful 

 and truly philosophical instrument, Mr. Daniell has left little chance 

 for improvement, so far as dehcacy and accuracy of indication are 

 concerned ; and were it less fragile, perhaps no other would be desired, 

 by those who experiment on aqueous vapor. As it is in mountain- 

 ous countries, or on the tops of mountains, that the results of experi- 

 ments on aqueous vapor, are of most value ; from the phenomena 

 being presented in a more simple form, and divested in part, of the 

 attendant circumstances which modify them ; an instrument adapted 

 to such purposes, should be as portable as is consistent with delicacy. 

 The simple apparatus devised Mr. Dalton, and used in those decisive 

 experiments, which have so intimately connected his name with the 

 history of meteorological science, is sufficient to enable us to deter- 

 mine the temperature of the aqueous vapor in the atmosphere, and 

 by a reduction in the size, and a slight modification of form, an instru- 

 ment is obtained, which is well fitted for general meteorological pur- 

 poses. Instead of the cylindrical jar used by Mr. Dalton, I substi- 

 tute a thin brass tube, the diameter of which, is one half greater than 

 that of the cylindrical, or spherical bulb of the thermometer, which 

 forms a part of the instrument ; its length is one inch and a half; it 

 is closed at one end, the other having a screw cut on the inside, to 

 receive a screw cap and a rim, or projection below, to allow the disk 

 of caoutchouc which precedes the cap, to be compressed, forming a 

 convenient and efficient " stuffing box." At that end of the tube 

 which fits the cap, a narrow ferule of polished platina, or steel is sold- 

 ered to the outside of the tube, and one half of its length from the 

 other end, is covered with soft cotton thread. The scale of the ther- 

 mometer, graduated from 120° to — 30°, terminates about one inch 

 from the bulb. At this extremity, a screw cap, and disk of caout- 

 chouc, both pierced for the tube of the thermometer, are firmly at- 

 tached. When not in use, the tube is closed by a cork, and packs 

 in the case, by the side of the thermometer and phial for containing 

 ether. 



In all aerological experiments, the thermometer is used for deter- 

 ming the temperature of the atmosphere, carefully avoiding those 

 sources of error, which render this a delicate operadon. For deter- 

 mining the dew point, so much alcohol is put into the tube, as is ne~ 



