of a New Hygrometer. 27 



water dissolves a saline substance, consequently all the 

 formulae for calculating the weight of vapour in a given 

 space have been made on this supposition, whereas if 

 the beautiful laws developed by the illustrious Dal ton be 

 applied to this instrument, I think every objection can be 

 fully and satisfactorily answered. 



Objections which have been advanced at various times 

 against the Hygrometer by evaporation. 



1st. That evaporation is considerably increased by cur- 

 rents of air. Thus the instrument would be affected by the 

 wind increasing the evaporation, and consequently depress- 

 ing the temperature, which would indicate a fallacious dry- 

 ness. 



2nd. That evaporation takes place with greater rapidity 

 in sun-shine than in shade, also indicating a fallacious dry- 

 ness. 



3rd. That the two thermometers are more or less influ- 

 enced by reflected light and heat, &c. 



4th. That evaporation is influenced by the density of the 

 atmosphere ; it being greater or less according to the height 

 of the mercury in the barometer. 



5th. That it is almost impossible to take the heat of the 

 air to any degree of nicety, without the observation being 

 affected by the power of radiation ; and if a radiant caloric 

 be allowed to interfere, the condition of calculation fails. 



6th. That more time is required, than could be bestowed 

 by an ordinary observer, in order to make a correct obser- 

 vation, as time must be allowed for the water to acquire 

 the same temperature as the atmosphere. 



\st. Objection. In answering the first objection it will 

 be necessary to enumerate some of the laws of evaporation, 

 which from being vaguely understood have always operated 

 to the disadvantage of hygrometers by evaporation. I need 

 not here enter into any minute analysis of the composition 

 of the atmosphere, as it would be foreign to my present 

 purpose. 



The two following laws will be sufficient to elucidate the 

 question. 



The first established by Mr. Dalton, " that gases, dry 

 atmosphere air included, act as vacua with regard to vapour, 



