whole energy may be used in propulsion alone. Now some may think they see in this a solution of the problem. They will say, "Why not sustain the machine by gas, so that the whole energy may be expended in propulsion alone?" I answer, that in proportion as the balloon principle is added to the flying principle, in the same proportion is size increased without corresponding increase in power; and therefore in the same proportion is increased the resistance of the air to propulsion, and, what is worse, in the same proportion is our machine at the mercy of winds. But it will be objected: "See the fishes, how they swim! They are not at the mercy of currents. They float suspended in the water—they dart forward against currents—they ascend cascades and leap waterfalls; in a word, they are largely independent of water-currents. Now suppose we make a machine exactly the shape of a fish, tail and all; then, by the addition of gas, make it the same specific gravity as the air; then, by machinery, make it wiggle its tail in the manner of a fish. Where is the difference? Why may we not make an aërial swimming-machine, if not a true flying-machine?" Doubtless it is in this direction that we must seek the partial solution of the problem, not indeed of flying, but of aërial navigation. Yet the answer to the extravagant expectations expressed above is plain. The fish—its bones, muscles, viscera, brain—the materials out of which are made machine, fuel, and engineer, are of the same specific gravity as the medium (water) in which it swims. Now, whenever we can find materials out of which to make our machine, fuel, and engineer, which shall have the same specific gravity as the air, then, indeed, we may make a successful swimming-machine which shall be independent of winds. But so long as our materials are six or seven hundred times (wood), or five or six thousand times (iron), as heavy as air, we shall not succeed, because of the enormous dead space filled with gas that we are compelled to use, which adds to the resistance of the air and the power of the winds, without adding anything to the power of propulsion.
Therefore, we repeat, a pure flying-machine is impossible. All that we can expect—all that true scientists do expect—is, by skillful combination of the balloon principle with the true flying principle, to make aërial navigation possible in moderately favorable weather—in other words, to make a locomotive balloon; or, if we choose so to call it, an aërial swimming-machine. That something really useful of this kind will eventually be made, there can be no reasonable doubt.