Of course an invention of such importance could not escape condemnation. Balloons were manifestly contrary to the will of Divine Providence, for, if it had been intended that man should fly, wings would have been given to him. Moreover, the barriers of virtue and

morality would be broken down by permitting aëronauts to descend into gardens and balconies; and, above all, the boundaries of empires would be practically annulled, and nations in consequence engage in continual war.

Well is it, then, for humanity that balloons have not proved a very great success. Many extensive voyages and many interesting observations have been made; but as a flying-machine the balloon has no place. It is the servant of the air, not the master. It must obey a will, pitiless, fickle, sometimes kind, but never trustworthy. The expectation that headway could be made against the wind by means of sails and rudders had no basis in sound theory or sense. A sailing-ship is immersed in two fluids of widely differing densities, and its sail is only effective because the water, while supporting, at the same time allows the vessel to move more readily in one direction than another.

FIG. 3.-SULLIVAN'S FLYING-MACHINE. (Taken from United States Patent-Office Reports.)

A balloon, on the other hand, is totally immersed in an ocean of air, and being of the same weight bulk for bulk, and subject to no external forces, must necessarily follow the slightest current. One might as well attempt to steer a boat, swept along by a great stream, without wind or oar. It forms an integral part of the current itself. It is a thistle-down blown by an autumn gale.

Of course we may provide our balloon with wings or propeller, and fly as the birds fly. This has been and continues to be a favorite combination with our inventors. One patented in this country in 1880 has been chosen as an illustration. The balloon, oblong in shape and divided for safety into compartments, supports a car containing the propelling machinery, and also a gas-generator to make up such loss of hydrogen as may occur. Two immense rudders steer the machine. It is propelled by four paddle-wheels, which would act, one would think, very much as the wheels of our river-steamers would act, if totally immersed in the water, and would be about as likely to drive the balloon backward as forward.

Generally, however, in machines of this class the propeller is one gigantic screw, or a number of screws, and the balloons have a variety in shape and grouping which is quite remarkable.

It is strange that people have not realized that a thing necessarily so big and light as a balloon can not be made strong and durable enough to stand the pressure of the wind at comparatively low velocities. Floating with the current, the velocity would have no destructive effect; but brought into opposition to this current, or forced at any great speed through the air, the resistance would be much greater than a silk bag could safely stand.

It may be well here to refer to a table giving the relation of pressure to velocity of air, experimentally determined and verified time and again-results very important in the study of flying and flyingmachines:

Now let us suppose that a balloon only forty feet in diameter should resist the pressure of wind blowing at the rate of twenty miles an hour, or, what is the same thing, that the balloon should be traveling through still air at this speed. The surface presented to the wind would be about twelve hundred square feet, and the pressure on each square foot, from our table, would be 19 pound, and the total pressure over a ton. A calculation is hardly necessary to show that such a pressure,

acting constantly upon our silk, would be likely to rupture it; and when we consider that sudden gusts might readily increase the pressure five-fold, it will be admitted that terra firma would be decidedly safer, if less exciting.

More than all this, balloons as hitherto constructed are at best but temporary affairs, quickly losing their gas and buoyancy, expensive and unwieldy, and, however valuable for certain kinds of work, must be considered as simply floating, not flying machines. If we expect to gain the respect of the birds or butterflies, we must go to work in a much less clumsy way.

In the excitement following Montgolfier's invention, simple flyingmachines dropped out of sight almost entirely, and it was only after a long series of disappointing trials that the old ideas came to the surface again. The balloon craze, however, brought about a more careful study of aëronautics generally; but at the same time there has been and is a strong current of misguided thought and invention, particularly to be noticed in our Patent-Office reports.

Inventors of flying-machines, as a rule, belong rather in a lower class. Just as we still find old-new arrangements for producing perpetual motion, so in the attempts to fly the old story is repeated. The perpetual-motion man is likely also to know just how to make a successful flying-machine. He only lacks the means. Still, particularly in England and on the Continent, many able men have been working intelligently, perseveringly, quietly. Before building a flying-machine they have thought best to study the examples Nature has provided, thinking that, while we need not necessarily imitate the mechanism, we may in this way get a better idea of the principles and action involved.

The broad principle governing either natural or artificial flight is quite simple, but the difficulty of applying it very great. Our flyingmachine, one that is much heavier than the air, and depending entirely upon its own power, in the first place, must be able by acting on the air to lift itself, and, while maintaining a position at any desired height, to propel itself forward. It must be prepared to encounter and take advantage of, and overcome currents of air sometimes hardly perceptible, sometimes perhaps a roaring gale-currents, too, not unlikely to suddenly change both in direction and velocity. It should be able to fly continuously for a long while, and should be tolerably safe.

On the water, if the machinery gives out, we can float or swim; but in the air any little difficulty of the sort would be likely to end unpleasantly. And even if, like a parachute, the machine could be made to drop slowly, in a brisk wind the final landing-place would for a while be a matter of uneasy conjecture.

It may easily be understood, then, that the problem is not a simple one, and yet, to a person watching, for example, the flight of a flock of gulls following in the wake of a steamer, the exquisite ease and grace and apparent simplicity of the movement are very striking. Sweeping

around in circles, occasionally elevating themselves by a few flaps of the wings, they glide down and up the aërial inclines without apparently any effort whatever. But a close observation will show that at every turn the angle of inclination of the wings is changed to meet the new conditions. There is continual movement with power-by the bird it is done instinctively, by our machine only through mechanism obeying a mind not nearly so well instructed.

The study of the flight of birds and insects has of late years received a great deal of attention, and, in a general way, the motions of the wings are fairly well understood. We could probably very closely imitate these motions, but the question at once arises, in doing so, would we be applying our power in the most effective way? While somewhat similar, the movement and construction of the wings of flying creatures vary considerably. What is best for a heavy body with short wings is by no means best for a light body with long wings; nor does a sea-bird, constantly on the wing, but perhaps not a rapid flier, fly in the same way as a pigeon or humming-bird; and, in any particular case, it does not necessarily follow that Nature has provided the most efficient apparatus; or, in other words, that the power the bird possesses could not be utilized more effectively. Nature can not always be trusted. We can study and understand her laws, but she does not pretend to apply them on economical principles. Fish and marine animals swim in a great variety of ways, they have all sorts of propelling arrangements, but there can be no doubt that a screw-propeller is vastly more efficient than any of them; and why should we try to copy the motions of a bird's wing any more than those of a fish's tail? The motions are very complicated in any case, and our machine, imitating them, would be complex and liable to get out of order. And one can not help thinking that we are about as likely to make a steam road-wagon by imitating the action of a horse, as we are to make a practicable flying-machine by copying the motions of a bird. The desired results can probably be obtained in a much more simple and effective way.

Still, the study of flying creatures has brought out many interesting and suggestive facts, and has given us, too, some encouragement.

In the first place, we notice that all birds are heavy, and that the expanse of wing generally diminishes in proportion to the increase of weight. The following is a table prepared by M. Lucy, showing this very clearly:

Table giving the Expanse of Wing-Surface for each Pound of Weight.