Proposed Improvement of the Hygrometer. 41 



1 2 glass vessels, in the first of which we mix 12 parts of this 

 spring water with one part of water which has acquired the 

 temperature of the air; in the second, 1 1 parts of the former 

 with two of the latter, and so in a decreasing ratio through- 

 out, we then obtain fluids which differ from each other by 

 one degree of temperature. Of these we examine the first 

 which does not bedew, and its temperature then precisely 

 indicates the expansive capacity of the water-gas of the air. 

 Should the air happen to be too dry, so as not to yield a 

 deposition of water, then we might render it cooler by the 

 addition of sal ammoniac which dissolves in it ; and in winter 

 we can increase the cold by snow, with salt or sal ammoniac. 

 But even this method is slow and troublesome for hygro- 

 metrical investigations. I therefore determined to precipi- 

 tate the water from the air by the thermometer itself, as 

 follows : 



The bulb, defended by a case of oiled silk, being immersed 

 in the cold water, was taken up after having acquired the 

 temperature of the water. It then became covered with va- 

 pour of water; I observed also the degree of the thermometer 

 when the dew disappeared, and found the expansive capa- 

 city of the water-gas, according to Dalton's tables, tolerably 

 near. But I also found that a very damp air at -f 18° to 

 -f- 20° produced a somewhat greater result than what it 

 ought to be, because the thermometer, when much water 

 is deposited on its bulb, proceeds little beyond the true point 

 before all is evaporated. Besides, there is another circum- 

 stance which renders the use of the ordinary thermometer 

 less certain, that, namely, a small portion of deposit is not 

 accurately distinguished on the bulb. I therefore caused a 

 thermometer bulb of steel to be made, greatly oblongated, 

 its outside highly polished, and thereto attached a steel 

 tube an inch long, wherein I closely cemented a thermo- 

 meter tube, and made thereof an actual thermometer. This 

 instrument completely answered my purpose. When the 

 bulb with its oil case was immersed in the cooling mixture, 

 and taken up as the mercury fell an inch, or every other 

 inch according to circumstances, I at last arrived at a pe- 

 riod when the bulb became covered with a quickly-passing 



coat 



