DIFFUSION. 



(vitiating the two values we obtain 



Here A i* l ^ e pressure f ^ e vapour in the atmosphere. The tempi- ra- 

 turr for which this is the maximum pressure is the dew-point, and p^ is the 

 maximum pressure corresponding to the temperature 0, of the wet bulb. Hence 

 thin formula, combined with tables of the pressure of aqueous vapour, enables 

 to find the dew-point from observations of the wet and dry bulb thermo- 

 metan. 



We may call this the convection theory of the wet bulb, because \\c 

 consider the temperature and humidity of a portion of air brought from a 

 distance to be affected directly by the wet bulb without communication either 

 >t' heat or of vapour with other portions of air. 



Dr Everett has pointed out as a defect in this theory, that it does not 

 explain how the air can either sink in temperature or increase in humidiu 

 unless it comes into absolute contact with the wet bulb. Let us, therefore, 

 consider what we may call the conduction and diffusion theory in calm air, 

 taking into account the effects of radiation. 



The steady conduction of heat is determined by the conditions 

 0=6, at a great distance from the bulb, 

 6 = 6 l at the surface of the bulb, 

 V'# = at any point of the medium. 



The steady diffusion of vapour is determined by the conditions 

 P=p t at a great distance from the bulb, 

 p=pi at the surface of the bulb, 

 V'p = Q at any point of the medium. 



Now, if the bulb had been an electrified conductor, the conditions with 

 respect to the potential would have been 



F=0 at a great distance, 

 VV l at the surface, 

 V*F=0 at any point outside the bulb. 



