434 



" If t and /' be the temperatures shown by a dry and wet 

 thermometer, encompassed by atmospherical air, t 1 ' the dew- 

 point,/' and/" the forces of aqueous vapour at t' and t", 

 and p the existing pressure, — I have shown (Trans. R. I. 

 Academy, vol. xvii., p. 285,) that 



fn-f, t'-t' Y P-f' 



J ~ J 87 80 • 



" In investigating this expression, it is assumed that the 

 specific heat of air, and the caloric of elasticity of aqueous 

 vapour, are constant, and represented (within the ordinary 

 variations of atmospheric temperature and pressure,) the 

 former by . 267, the latter by 1 115. In subsequently apply- 

 ing this expression to the determination of the specific heats 

 of the gases, (Trans. E. I. Academy, vol. xviii.) it was ne- 

 cessary to give it its most general form, when it was found 

 to become 



48a(<- Q p-/'. 

 J -J * e x_ 30~' 



a being the specific heat of air, and e the latent heat of 



aqueous vapour, both being supposed at the temperature 



represented by t', and under the pressure p. I shall here 



briefly indicate the steps which conduct to this result. 



They are given at length in the Philosophical Magazine, for 



October, 1838. 



" The two following propositions constitute the basis of 

 the investigation : 



" 1st. When, in the case of the wet thermometer, the 

 stationary temperature is attained, the caloric which vapo- 

 rizes the water, is necessarily equal to that which the sur- 

 sounding gas evolves in descending through t — t' degrees, 

 i. e. from the proper temperature of the air to that of the 

 moistened bulb. 



" 2dly. The air so cooled, by successive contacts with 

 the moistened bulb, is saturated with humidity. 



" From these propositions we easily deduce the equation 



