truth is that any machine may become superannuated either owing to depreciation or to obsolescence; in the former case the number of miles covered will be the determining factor, coupled, perhaps, with other facts relating to its history or usage; in the latter, it is the age of the machine which determines its unfitness for service, considered, of course, in relation to the advance which has been made in the art of construction since the date of building. Thus a machine may be unfit for service either because it is, according to some accepted definition, worn out and incapable of repair, or because it is obsolete in design. In some cases obsolescence may be absolute, as when a design is so out of date that by comparison with the best available it is to be considered unsafe or uneconomical; in this case it is only fit to be destroyed; or its obsolescence may be relative, as when it is out-classed by the machines of corresponding type in the service of some neighbouring Power; in this case it is fit to be sold out of the Service or to be transferred to some distant part of the Empire, where competition is not equally severe. The questions of depreciation and obsolescence and the disposal of condemned machines have not yet received due consideration. On continuous active service it would appear that the life of a present day aeroplane is about three or four months.
The foregoing may be taken merely samples of the many questions which have to be faced before the training of army pilots and aeroplane gunners and signallers can be attempted in the thousands, or tens of thousands, for which the warfare of the future may call. Without adequate consideration of these questions, coupled with appropriate measures, progress in the direction of increase of numbers and the practical development of aeroplane tactics on a large scale will be most seriously handicapped.