Scarcely a generation has passed during the evolution of the airplane from a ridiculous dream to a practical factor in the work of the world. Men who once read with derision, or only passive interest at best, of the experiments of Langley, Chanute, and the Wrights have seen the airplane developed suddenly into an indispensable instrument of war and an agency of demonstrated value and of such diversity of application that its future is hard to estimate. The navigation of the air has accomplished much in many fields. Not only does it offer a new means of efficiency in military reconnaissance, rapid delivery of mail, fire patrol of forests, and the constantly increasing number of commercial and scientific pursuits to which it is being adapted; but it has also opened a new world to the geographer, the physiographer, and the geologist. AIRPLANE PHOTOGRAPHY: ITS DEVELOPMENT AND APPLICATION Very early in the war the airplane was recognized as a useful, in fact a necessary, means of observing enemy positions and movements. But the speed of the airplane was found to preclude the taking of more than the most hurried of notes during a flight, and notes written from memory are not the most satisfactory. Photography was found to obviate this difficulty. The ability of the camera to make instantaneous exposures and fix a clear image on a photographic plate enabled the observer to obtain a record not only of the scenes that he had viewed but also of many that he might have missed while engaged in the necessary business of watching the sky for the enemy--a record that for detail and accuracy could not be approached by the most elaborate notes or the most graphic description. Immediately inventive genius was set at work to adjust the mechanism of the camera to the demands of air photography and to prepare the rapidly working films and highly sensitized paper necessary for the best results. So satisfactory were the results and so great are the possibilities of further adaptation that there is an unfortunate tendency on the part of certain enthusiasts to make exaggerated claims that may react to retard progress. This is particularly true in the use of the air photograph in mapping. There are limitations to this use of air photography. It cannot be reasonably expected to do away entirely with the ground work of the surveyor. Rather, the camera is to be regarded as one of the instruments of the surveyor. Observation from the air can never take the place of close examination of the ground, but it can be of great use in the location and study of land forms and geologic relations. Air photography is only an added means of obtaining information, although it promises to become a very important means.