The pressure differences between the top and the bottom surfaces of the weighing container represent average, macroscopically observable, behavior of the molecular ensemble. Using equation (9) these effects can be interpreted microscopically to result from differences in collision frequency, if, in the usual manner, we imagine the gas as a collection of hard spheres in constant motion. At any height y the upward
flux of molecules leaving the volume = VA y ddwhose midpoint is y with vertical velocity (magnitude) vy is given by