It is interesting to note that gravity in this context tends to cause a drift of momentum downward or a bias towards more forceful collisions on the bottom of the container than on the top for an individual molecule. This is hardly counterintuitive and if the various trajec- tories are treated collectively is the physical basis for a lower density with increasing height. This is similar to the idea of drift velocity in electricity, where charge is transported due to the bias introduced by an electric field.