the Dew-point by the Wet-bulb Hygrometer. 269 



tion. Such is the principle of the test experiments which I first 

 performed. The air was heated by urging it in a continued 

 stream by means of a double bellows through the worm of a 

 small still, such as are for sale in the opticians' shops, the 

 worm-tub being filled with water of the desired temperature ; 

 and, in order to the necessary observations, in a glass tube, 

 connected by a cork with the upper extremity of the worm, a 

 couple of small thermometers were placed, their bulbs being 

 separated by about a quarter of an inch, and that of the in- 

 strument occupying the higher position being invested with a 

 tunic of muslin kept constantly moist with water. The blast 

 was steadily maintained until the thermometers ceased to rise, 

 and the temperature of each was then accurately noted, the 

 eye being assisted by a lens. Tables I., II., III., and IV. 

 exhibit the results of four distinct series of experiments thus 

 conducted. 



Table I. 

 February 8, 1835. 11 o'clock a.m. 



Table II. 

 February 9, 1835. 11 o'clock a.m. 



47-2 

 76- 



42-5 

 57-5 



4-7 



18-5 



30-02 

 3002 



t" obs. t" calc. 



38 

 38 



36-58 

 40-44 



Diff. 



-1-42 



+2-44 



Table III. 

 March 4-, J 835. 1 1 o'clock a.m. 



