Mr Foggo on the Dew-Point Hygrometer. 37 



the present opportunity of making some. remarks on dew-point 

 instruments in general. 



The superiority of these instruments over hygroscopic sub- 

 stances is derived from the circumstance, that they are not 

 hable to deterioration from use or time, and they possess the 

 further advantage, that their indications are strictly compa- 

 rable, being independent of every change in the condition of 

 the atmosphere, excepting those alone which they themselves 

 are employed to detect. 



The idea of ascertaining the hygrometric state of the air by 

 cooling a portion of it till its moisture was rendered visible, ap- 

 pears to be the oldest as well as the simplest, and was perhaps 

 suggested by observation of phenomena upon the great scale of 

 nature. 



The Florentine Academicians employed a glass vessel of the 

 form of an inverted cone, which they filled with ice ; and they 

 estimated the degree of dryness or humidity by the frequency 

 of the drops formed by the trickling down of the dew deposit- 

 ed from the chilled air in contact with the sides of the glass. 

 Their experiments, however, were made at the very dawn of 

 the sciences ; and the other branches of natural knowledge 

 were too little advanced to assist them in drawing any useful 

 consequences from them. We now know from the laws which 

 regulate the condition of vapour, that the frequency of the drops 

 would indicate only the changes in the density of the vapour 

 as it varies in warm and cold seasons, and not the relative dry- 

 ness or humidity of the atmosphere at the time of the experi- 

 ment. M. le Roi adopted a method susceptible of more pre- 

 cision, though even in his time the relations which his results 

 bor6 to the actual quantity of moisture in the atmosphere were 

 not understood. He filled a glass vessel with water, and low- 

 ered its temperature by stirring bits of ice in it till the cold 

 was sufficient to condense the moisture. He then noted the 

 temperature at which the precipitation first began, and judged 

 of the degree of humidity by the difference of this temperature 

 and that of the air. Since his time Mr Dalton has made many 

 thousand observations nearly in the same manner, but by cool- 

 ing the water by an artificial saline mixture instead of ice. He 

 uses a cylindrical glass jar, dry on the outside, which he fills 



