The drag which the earth's surface exerts on the wind has been measured directly by observing the deflexion of a horizontal plate, floating in oil and under torsional control, and forming part of the earth's surface. Simultaneous measurements of the velocity profile above the surface have enabled von Karman's constant for the lower atmosphere to be deduced. The value obtained under moderately unstable atmospheric stratification is 0$\cdot $46, which is considered to be in good agreement with Nikuradse's value of 0$\cdot $40, obtained from the flow of water through pipes under isothermal conditions. Evidence is also adduced for a considerable variation of von Karman's constant with atmospheric stability. The observations of drag and velocity profile have also been used to determine the variation of mixing length with height. In unstable conditions the mixing length increases more rapidly than the height, while in stable conditions the increase is likely to be less rapid than the height.