Neither the diffusional theory of Maxwell nor the convectional theory of August is capable of representing adequately the processes of evaporation and heat transfer occurring at the wet bulb. An examination of these processes with the aid of the Reynolds analogy between heat and vapor transfer by convection on one hand, and fluid friction on the other, indicates that the correct theory consists of a combination of the two older theories, in which a mechanism of convection plus conduction is postulated for both heat and vapor transfer. When the thermal diffusivity is equal to the material diffusivity, a special case occurs, for which all three theories give the same result. This condition is approximately satisfied for the case of water evaporating into air; other gases or liquids must therefore be used to discriminate among the various theories. Experiments with water and four organic liquids show that the new theory represents the facts very accurately, and may be made the basis of a sound system for the reduction of psychrometric observations.