Man will yet learn how to fly. Professor Langley, secretary of the Smith sonian institution, so declared in an ad dress delivered before the Nationa Academy of Sciences. This ought t give much encouragement to thinkers who hope that mankind will some da; eecnre domain in the element of birds, jnst as it has already done, thanks to the evolntion of snbniarine boats and diving gear, in that of fishes. The distinguished scientist referred to expressed confidence that the contrivance of the fnture for aerial navigation would be on the kite principie. The air possesses elements of buoyancy which have not been recognized hitherto. There is no truth in the popular conception that a body heavier than the atmosphere cannot be suspended in that medium withont motion. A kite of sticks and paper is much heavier than the fluid which it displaces, but it is sustained aloft. You can find an example much inore striking, however, in the eagle or the frigate btrd, which, though an animal of considerable weight, remains poised in the sky upon extended pinions inotionless for hours together, so that a telescope inay be trained on it. Thousands of feet above the earth it is sustained without inovement of a feather, though in a rarefied atmosphere. This ia possible because the suspended fowl has au instinctive knowledge of the way in which to utilize the air currents for its support. When man has learned how this can be done he will be able to fly. The inost advanced investigators in this subject reject the balloon and all other snch lifting devices as impracticable; a gas lighter than air can never be safely confined within a receptacle that is not weighty, and the same objection applies to a vacuüm. Ballooning today is about where it was when it was first invented, and in the nature of things it never get much further. Aluminnm is light for a metal, but it is severa! times too heavy to be successfully utilized for such mrrooses. Pray, what has become of those aluminum trains of cars that were to be run tkrough the air froin Chicago to New York at sixty minutes the trip? LESSONS FROM BIRDS. There is a school of flying machine inventors who may be designated as the "flappers," inasmuch as their idea is to sustain their contrivances by the flapping of birdlike wings. But they doubtless forget that the best flyers among birds do not support themselves by flapping. On the contrary, they only resort to that performance when it is necessary for a start. The eagle, if launching himself from level ground, is obliged to take quite a run first; then, flapping his wings with a violent muscular effort which he coultl not keep up for long, he gains a sufflcient altitude to render it possible for him to strike along the plane of an air current which holds him up. Floating with librating pinions from one air current to another, he is lifted, with an occasional broad sween of his powerful wings, to the upper aerial regiems, where he simply floats, opposing to each movement of the suspended ether the gentle force necessary to maintain him in his stationary position. These principies which the eagle applies for purposes of flight are perfectly understood in the theory of mechanics. It is only necessary to adapt them with suitable apparatus in order to give to human beings like powers. True, the bird is lighter in proportion to its size than is man; its bones are hollow and filled with warm air from the lungs. But the difforence in this .respect is not very material, and it may easily be compensated for by bigger wings. Power to flap them is not what is required, inasmuch as the start can be made from a height; what is wanted is the knowledge which inherited experience - "instinct"- has given the fowl as to how to adapt the angles of the wings to the air currents. A PRACTICABLE MACHINE. This is precisely what Professor Langley has been experimenting with. What he is attempting is to produce a machine adapted to flotation upon the air currents like a kite. You can ñnd a very simple illustration of the principie he is working on in the trick ilone with ing carus Dy tne prestidigitator Herrmann, who throws thern f rom the stage into the highest gallery of the biggest theater in the United States, distributing them one after another among the "goda" of that select circle. How does he do it? Simply by skillfully utilizing the air currents. The scrap of pastboard is heavier than the atinosphere, but, judiciously projected, it monnts to a great height and distance with very little force. It would stay up, too, even if thrown outdoors, supposing that it possessed the intelligence necessary to accoinmodate its surface to the winds. What can be done in this way may be accomplished on a larger scale. Lest this proposition be disputed it will be suffieient to refer to a flying machine patented in Australia. It weighs altogether nineteen pounds, and its backbone is a long copper cylinder two inches in diameter, filled with compressed air. The compressed air supplies power for a small engine weighing ten pounds, at the rear end of the cylinder, which works a fan propeller. To the backbone on either side is attached a great wing of light material, so that the whole apparatus resembles abig butterfly wth two fans for a tail. This contrivance has been made to fly horizontally 360 feetv Of course it is only a beginning, but all the same here is the nearest approxich to the practicable flying machine of the future thus f ar taiued.