1836.] of the Wet-bulb Hygrometer. 417 



Now in these instances the evaporation certainly followed the in- 

 verse pressure law ; but the depression was made to receive only a con- 

 stant arithmetical increment for each geometrical decrement of the 

 pressure ; in accordance with which I assumed that the proper correc- 

 tion for variation of pressure should be d , — rather than d — ; and 



v p p 



even this would require a different co-efficient to make it suit the two 

 cases quoted above. Under such an uncertainty as to the real amount 

 of this important correction, I was induced to direct a fresh series of 

 experiments to this particular object ; and as my results differ greatly 

 from what has preceded, it is incumbent on me to describe my process 

 a little in detail. 



I first prescribed to myself the necessity of working with a current 

 of air as similar as might be to that of the maximum series, as with- 

 out such a precaution it would be impossible to ensure the permanent 

 hygrometric status of the air in contact with the wet-bulb. The 

 bell glass of an air-pump, under which I imagine the experiments of 

 Daniell and Anderson to have been conducted, could not possibly 

 fulfil this indispensable condition, since a partial halo of moisture 

 would encircle the bulb of their thermometer ; — nor do they appear 

 to have used a hair hygrometer to inform them how far this might be 

 the case. Mr. Daniell it is true had a dew-point instrument fitted 

 into the side of the glass receiver, but for slight aqueous tension this 

 instrument becomes wholly useless. The extent to which his air was 

 dried can be calculated pretty well from his own datum that the 

 depression at 50° was nine degrees, which by my table would indicate 



centesimal tension "30 : or by Apjohn's formula * ' "^ — ' = -42 



J .357 



in the latter case requiring a cold of 8 degrees, and in the former of 



16, below the freezing point to produce deposition. 



But to return to my own experiments : — 



In place of the short open glass tube connected with the gaso- 

 meter and glass balloon in which the wet-bulb was before exposed 

 to the current of air, (fig. 1,) a thin horizontal brass tube (fig. 7) 

 was substituted, having two lateral apertures for the admission 

 through corks, air-tight, of the dry and wet thermometer bulbs 

 (t, t'). From the same brass tube descended a glass barometer tube 

 (p) into a reservoir of mercury, similar to the gage of an air- 

 pump, for marking the actual pressure close to the thermometers. 

 The other end of the tube was conducted by a flexible pipe F to the 

 receiver of an air-pump, where a continual vacuum could be kept up 

 by pumping without intermission during the course of an experiment. 

 3 H 



