From this description of its action, it will be seen that the Pénaud tail has the disadvantage of giving an undulatory flight, if the tail is made rigid. This objection, however, can be easily overcome by giving to it a certain amount of elasticity. It does not appear that Pénaud gave much attention to this feature, but stress is laid upon it in the article “Flight,” in the ninth edition of the Encyclopædia Britannica, and I have introduced a simple device for securing it. The complete success of the device implies a strictly uniform velocity and other conditions which cannot well he fulfilled in practice. Nevertheless, it is as efficient a contrivance for its object as has yet been obtained. More elaborate devices have been proposed, and a number of them, depending for their efficiency upon the action of a variety of forces, have been constructed by the writer, one of which will be described later. This has the advantage that it tends to secure absolutely horizontal flight, but it is much inferior in simplicity to the Pénaud tail. Apart from considerations about the thrust, the ‹CP› is in practice always almost directly over the ‹CG›, and this relationship is, according to what has been suggested, obtained by moving the supporting surfaces relatively to the ‹CG›, or ‹vice versa›, remembering, however, that, as these surfaces have weight, any movement of them alters the ‹CG› of the whole, so that successive readjustments may be needed. The adjustment is further complicated by another important consideration, namely, that those parts which ‹change› their weight during flight (like the water and the fuel) must be kept very near the ‹CG›. As the water and fuel tanks are fixed, it appears, then, that the center of gravity of the whole is practically fixed also, and this consideration makes the adjustment a much more difficult problem than it would be otherwise.[25] [Illustration: PL. 11 STEEL FRAMES OF AERODROMES NOS. 4, 5, 6. 1893, 1895 AND 1896] [p053] CHAPTER VII HISTORY OF CONSTRUCTION OF FRAME AND ENGINES OF AERODROMES