[D] “It is estimated that the average cost to France of training each pilot is five thousand dollars ... no less than from four to six months are devoted to the training of finished pilots. Although I have just come from France, the progress of aviation is so rapid that much of my own knowledge may be out of date before I again return to the front.”--C. D. Winslow, “With the French Flying Corps,” 1917, 4–5: N. Y. CHAPTER II APPLIED METEOROLOGY FOR THE AVIATOR _Activities of the Weather Bureau in Relation to Aëronautics._--Naturally the progress of aërial navigation has at all times been rather closely connected with the Weather Bureau. For over a decade the Bureau has not been content with surface observations but has maintained laboratories for the study of the upper air. The results of its observations are considered a mine of information for the student aviator. Prof. Charles F. Marvin, the Chief of the Weather Bureau, is a member of the National Advisory Committee for Aëronautics, and chairman of a subcommittee engaged on the determination of the problems of the atmosphere in relation to aëronautics.[E] [E] Monthly Weather Review, 1915, 32:500. Washington. The first official coöperation between the Weather Bureau and the War Department aviation school was inaugurated in the year 1914 by Dr. W. J. Humphreys, Professor of Meteorological Physics, when he was detailed to give a course of lectures. It was during this course that he lectured on “Holes in the Air.”[F] This paper has been reprinted as a textbook for the aviation school. [F] Popular Science Monthly, 1914, 44:18–34, N. Y.