1-It is now generally recognized that in a turbulent medium, such as the lower atmosphere, the processes of the diffusion of mass, heat, and momentum are dominated by the action of eddies in the wind. The exact mechanism by which the typical frictional and diffusion effects are set up by the turbulence is still obscure, and at the present time there appears to ire little hope of the presentation of a comprehensive theory of turbulent motion. The object of the present paper is to set forth a theory which, while admittedly non-exact and based partly upon an empirical assumption, appears to afford a satisfactory basis for the preliminary consideration of the allied problems of wind structure and the natural evaporation from a freely exposed plane surface in the lower atmosphere. 2-It is noticeable that most of the theories of turbulence which have been advanced treat the diffusion phenomena by means of a model which is suggested by the kinetic theory of gases. The eddies are regarded as distinct masses of fluid which behave like the molecules of the kinetic theory in that they are assumed to move along a hind of "free path," and thereby to transfer mass, heat momentum from one layer of the fluid to another by a process which is regarded as being substantially identical with that described by the collision dynamics of the kinetic theory. Briefly, it is assumed that a certain mass of fluid, of dimensions small compared with those of the total volume of fluid under consideration, breaks away from its surroundings under the influence of some mechanical or thermal disturbing force, and moves to another region of the medium, carrying with it a content of mass, heat, and momentum topical of the layer from which it originated. Having moved a certain distance (the Mischungsweg of Prandtl) the eddy is then conceived to mix instantaneously with the surrounding fluid, much in the same manner as a molecule transfers a portion of its energy when it comes into collision with another molecule.