A point of considerable importance, in regard to an air-corps, concerns the money which should be put aside, each year, for the provision of new machines. One eminently practical authority, Colonel J. E. Capper, reckons that, in connection with a fleet of a hundred aeroplanes, an allowance should be made for the purchase of forty new machines each year. Upon this question of renewals there is, however, diversity of opinion. The contention is made, for example, that a Government should be prepared, at the commencement of the flying season, to relegate all its previous year’s machines to the schools, for the use of pupils, and purchase a new fleet of up-to-date craft for use in war-time. Such a drastic step, however, should not be necessary. It would be advisable, of course, to weed out a number of machines, from time to time, for the reason that they become obsolete; and such machines should, as suggested, find a place at the schools for the use of beginners. The exact number of new aeroplanes which it should be advisable to buy, in any one year, must be governed, very largely, by the process of perfection which goes on. For the next year or so, it is probable that an allowance for renewals will need to be a heavy one. Afterwards, as the rate of improvement becomes slower, the purchase of new machines will represent a lighter item. A good reconnoitring biplane, say of the flying season of 1911, is not likely to become obsolete in 1912. A new machine will probably fly farther and faster, and carry more weight; but the 1911 biplane will still be capable of useful work, and need not be relegated to the scrap-heap. It will behove a Government, of course, to equip itself with as many new-type machines as possible; and an estimate of forty new machines a year, in connection with a fleet of a hundred, is by no means unreasonable. This, of course, presupposes a logical process of development, with an improved type of machine appearing from year to year. Should a revolutionary discovery be made, the plans of all nations would be altered. It might then become necessary, in the interests of national safety, to "scrap" a whole fleet of aeroplanes, in order to make way for the type which had made them obsolete. But the unexpected production of an aeroplane, immeasurably superior to existing models, is not anticipated. Already, it is true, the way can be seen to make many improvements upon present-type aeroplanes; but, in regard to such a difficult problem as that of aviation, the testing and perfecting of any new device, however simple, cannot be hastily carried out. One other consideration, in regard to the running costs of a fleet of machines, now presents itself. This concerns the allowance to be made for the general upkeep of the aeroplanes, and for such items as the provision of petrol and oil. Here an expert computation places the figure—for an air service of a hundred machines—at a sum of £16,000.