To return to our main theme, all should admit that if any hope for the flying-machine can be entertained, it must be based more on general faith in what mankind is going to do than upon either reasoning or experience. We have solved the problem of talking between two widely separated cities, and of telegraphing from continent to continent and island to island under all the oceans--therefore we shall solve the problem of flying. But, as I have already intimated, there is another great fact of progress which should limit this hope. As an almost universal rule we have never solved a problem at which our predecessors have worked in vain, unless through the discovery of some agency of which they have had no conception. The demonstration that no possible combination of known substances, known forms of machinery, and known forms of force can be united in a practicable machine by which men shall fly long distances through the air, seems to the writer as complete as it is possible for the demonstration of any physical fact to be. But let us discover a substance a hundred times as strong as steel, and with that some form of force hitherto unsuspected which will enable us to utilize this strength, or let us discover some way of reversing the law of gravitation so that matter may be repelled by the earth instead of attracted--then we may have a flying-machine. But we have every reason to believe that mere ingenious contrivances with our present means and forms of force will be as vain in the future as they have been in the past.