A flying machine must therefore be constructed to take advantage of these natural laws, to imitate the bird, “that admirable type of aerial locomotion.” according to Dr. Marcy, of the Institute of France. In short the contrivances likely to solve the problem are of three kinds:— 1. Helicopters or spiralifers, which are simply screws with vertical axes. 2. Ornithopters, machines which endeavour to reproduce the natural flight of birds. 3. Aeroplanes, which are merely inclined planes like kites, but towed or driven by screws. Each of these systems has had and still has it partisans obstinately resolved to give way in not the slightest particular. However, Robur, for many reasons, had rejected the two first. The ornithopter, or mechanical bird, offers certain advantages, no doubt. That the work and experiments of M. Renard in 1884 have sufficiently proved. But, as has been said, it is not necessary to copy Nature servilely. Locomotives are not copied from the hare, nor are ships copied from the fish. To the first we have put wheels which are not legs; to the second we have put screws which are not fins. And they do not do so badly. Besides, what is this mechanical movement in the flight of birds, whose action is so complex? Has not Doctor Marcy suspected that the feathers open during the return of the wings so as to let the air through them? And is not that rather a difficult operation for an artificial machine? On the other hand, aeroplanes have given many good results. Screws opposing a slanting plane to the bed of air will produce an ascensional movement, and the models experimented on have shown that the disposable weight, that is to say the weight it is possible to deal with as distinct from that of the apparatus, increases with the square of the speed. Herein the aeroplane has the advantage over the aerostat even when the aerostat is furnished with the means of locomotion. Nevertheless Robur had thought that the simpler his contrivance the better. And the screws—the Saint Helices that had been thrown in his teeth at the Weldon Institute—had sufficed for all the needs of his flying machine. One series could hold it suspended in the air, the other could drive it along under conditions that were marvelously adapted for speed and safety. If the ornithopter—striking like the wings of a bird—raised itself by beating the air, the helicopter raised itself by striking the air obliquely, with the fins of the screw as it mounted on an inclined plane. These fins, or arms, are in reality wings, but wings disposed as a helix instead of as a paddle wheel. The helix advances in the direction of its axis. Is the axis vertical? Then it moves vertically. Is the axis horizontal? Then it moves horizontally. The whole of Robur’s flying apparatus depended on these two movements, as will be seen from the following detailed description, which can be divided under three heads—the platform, the engines of suspension and propulsion, and the machinery.