136 Introduction to Boileaus [No. 147. 



1. That the specific heat of air, and the caloric of elasticity of aque- 

 ous vapour are constant, and represented within ordinary variations of 

 atmospheric temperature and pressure, the former by the number '267, 

 the latter by -1115. 



2. That where a dry and moist bulb Thermometer are exposed to 

 the influence of the same atmosphere, when the latter has obtained a 

 stationary temperature, the caloric which vaporizes the water is equal 

 to that which the surrounding gas evolves in descending through that 

 number of degrees at which the moist bulb stands below the dry, i. e. 

 from the proper temperature of the air to that of the moist bulb. 



3. That the air so cooled by the successive contacts with the mois- 

 tened bulb is saturated with humidity. 



If now a represent the specific heat of air, 



e the latent heat of aqueous vapour, 

 t and t' the observed temperatures of a dry and wet bulb Thermo- 

 meter encompassed by atmospheric air 



t" the observed temperature of the dew-point, 

 /and/" the elastic forces of aqueous vapour at t and t' 

 p the existing pressure in inches and decimals, 

 30 a standard Barometer pressure in inches, 

 then the general expressions for the force of atmospheric vapour at 

 the temperature of the dew-point in terms of the force of vapour at t' 

 and of the difference of the temperatures of the wet and dry Thermo- 

 meter are where t' is greater than 32° Faht. 



and where t' is less than 32 F. 



_ ,, 43 a(*-f) p-f (II) 



/ — J — e A 30 



in which by substituting for a, the value assumed above -267 and for 

 e its value at 50° upon the hypothesis that '967 is the latent heat 

 of vapour at 212° and that the sum of the sensible and latent heat is 

 at every temperature a constant quantity. 

 Equation (I) becomes 



/"=/'_- 01135 (f-OXif - - (HI) 

 and Equation (II) becomes 



/"=/' — 01017 (t—f) X ^4q - - (IV) 



