58 Mr Foggo on the Deto- Point Hygrometer. 



with cold spring water fresh from the well. If dew be imme- 

 diately formed, the water is poured out and allowed to stand 

 some time, in order to acquire heat, and is then replaced into 

 the jar, after the outside has been well dried with a linen cloth. 

 This operation is to be repeated till dew ceases to be formed, 

 and the temperature of the water is to be observed. It was 

 found that spring water generally answered the purpose during 

 the three hottest months of the year, but in the other months 

 it was necessary to employ a cooling mixture. Mr Dalion can 

 thus ascertain the dew-point to one-fifth of a degree of Fah- 

 renheit's scale. I may observe, that it is more convenient to 

 have two or more small cylinders of polished metal ; (about two 

 inches deep and one inch in diameter;) one of these is to be filled 

 with cold water, and, if dew be formed, the water is to be pour- 

 ed from one to the other, the warmth of the hand soon raising 

 the temperature sufficiently to drive off the dew ; or if it be ne- 

 cessary to use a saline frigorific solution, a small quantity of a 

 mixture of equal parts nitrate of potash and muriate of ammonia 

 may be thrown into it, and after the experiment the salts may 

 be recovered by evaporation. Mr Dalton was the first who 

 could deduce any important conclusions from observations of 

 this kind. Having previously determined, by a series of ad- 

 mirable experiments, the elastic force of vapour for a long range 

 of temperature, he was thus enabled to discover how much of 

 the pressure of the whole aerial column was to be attributed to 

 the presence of the moisture blended with it. This was a 

 great step in the advance of hygrometry ; and being followed 

 shortly afterwards by the investigations of Gay-Lussac on the 

 expansion of gases and the density of vapours, the whole 

 theory of this science has been established by the mathemati- 

 cal demonstrations of Biot. It has been shown, that, to ascer- 

 tain the condition of the atmosphere with respect to moisture, 

 it is necessary only to know its temperature, and that of its 

 dew-point, or the temperature at which its moisture begins to 

 be condensed. But the methods I have described of obtain- 

 ing this last term is evidently so troublesome, that it has never 

 been introduced to any extent. In the meantime, meteorolo- 

 gists have had recourse to the principle of evaporation pro- 

 posed by Dr Hutton, or to the hygrometers of Saussure and 



