In the course of government-sponsored work in the development of the constant-level balloon, the writers supervised many constant-level balloon flights on the standard meteorological surface of 300 millibars. The tracks of these flights are of general meteorological interest, as they are believed to delineate the actual trajectory of an equivalent mass of air travelling on a constant-pressure surface. Some of the original flight data are presented for the information of other investigators of air motion, and to illustrate the application of constant-level balloons to meteorological problems, particularly atmospheric diffusion and the variability of the wind. Suggestions are made regarding the significance of the data presented, and further experiments are proposed which might result in a better understanding of the mechanisms of air motion. Possible applications of constant-level balloons in routine observations, to obtain representative measurements of wind velocity, are set forth.