distances are so general as to ensure a free state and crossing the Marne and in frequent reports by the artillery of accompaniment. For the navigation of air- craft of the Amer, Soc. C.E., vol. 88, pp. 462-471 (1913). 6 Proc, Inst. C. E., vol. 43, p. 384 (1924); I. W. Green and K. H. J. Müller and C. J. H. Thomas