The device by which Mr. Henson has gained so great an additional likelihood of success, applies, not to the construction of the machine, but to the manner of using it. The carriage, loaded and prepared for flight, starts from the top of an inclined plane, in descending which, it acquires the velocity necessary for its further flight. The mode in which that velocity sustains it in the air is readily understood: the machine advances with its front edge a little raised, so that its under surface impinges obliquely on the air: that impact is accompanied by a resistance of the air, which is sufficient to prevent the descent of the machine; just as the wind striking the sails of a windmill obliquely presented to it, has power enough to propel them with all the machinery they set in motion.

So far, then, it seems that the velocity gained in descending the inclined plane, is that by which the machine proceeds and is sus tained, and, but for hindering forces, would proceed for ever; for it is a mechanical axiom, verified by all the results of art and science, that if hindering forces could be taken away, a body once set in motion would move for ever. But this motion through the air, though of itself it generates the perpendicular resistance of that fluid by which the machine is sustained as to elevation, generates also at the same time a resistance in the forward direction by which in no long time the motion itself would be destroyed, and the machine brought to the ground. Now it is to repair this decay of speed, to restore every instant the velocity lost in that instant, that the small steam-engine embarked in the machine is alone wanted, and it is easy to see that the power required for this effect must be very much less than that which would be necessary to lift and to start the machine; the entire amount of which power, it has hitherto been supposed, the machine itself must carry.

The great novelty, then, of Mr. Henson's aerial carriage, and the very important advance its inventor has made towards success in this oft-defeated enterprise, is the separation of the starting from the maintaining power. Although this is no novelty in abstract science it produces all the effect of a most important invention in its application to this purpose; and it is no slight ground for believing that Mr. Henson will eventually succeed, to find that his chief novelty accords so exactly with established science: as far as this device is concerned there is nothing whatever which can raise a doubt.

Familiar, however, as this principle may be to those in any degree accustomed to mechanics, its importance in this extraordinary design requires that it should be carefully illustrated. The weight of a clock is never able to set the clock in motion; but when the pendulum has been made to swing by being drawn out of the pendicular, the weight amply suffices to keep up its motion. Nor would even the weight be needed but for the resistance of the air and the friction and stiffness of the machinery by which the motion of the pendulum is registered and indicated:

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these destroy a minute part of the pendulum's motion at every vibration, which destroyed part it is the office of the weight to restore. The pendulum really moves by virtue of the force first exerted in drawing it from the perpendicular: the weight prevents the decay of that force. Now just this takes place with Mr. Henson's machine: it is set in motion by its descent down the inclined plane; it is kept in motion by the steam-engine it carries.

In nature the same process may be observed. A crow in rising from the ground is under the necessity of making very strenuous efforts with his wings to lift himself: while doing so he acquires horizontal velocity, and as soon as that velocity is sufficient to bring the resistance of the air to act on his sloping front and wings with effect enough to sustain him, he proceeds with comparatively easy beats; after a time we may see the same bird quietly sailing round and round in the air, scarcely moving his wings at all. Many of our readers must have asked themselves how a bird with merely outstretched wings is kept from falling? They will now readily see that it is by virtue of its original velocity, maintained and perhaps augmented in former parts of the flight.

But further it will be observed that it is horizontal velocity which is required, and that is gained by Mr. Henson in descending an inclined plane. Now just this device is often employed by large birds in starting from an eminence: instead of incurring the great labour we have noticed in the case of the crow, the feathered voyager makes first a curve downwards, the velocity gained in which, with subsequent and easy augmentations, is that which keeps up his flight. It is not often that a new contrivance in art has so exact a prototype in nature.

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The steam engine invented by Mr. Henson to meet the especial ne cessities of his aerial carriage, is distinguished by its extreme lightness in comparison with its power. This is effected, in great part, by reducing the necessary weight of water. The boiler mainly consists of a considerable number of inverted cones, presenting their blunted points and much of their surface to the fire. The amount of surface acted on by radiating heat is about 50 square feet, and about as much more is exposed to the heat of communication. paring the boiler with those of locomotive engines, it is expected to furnish a quantity of steam equivalent to the power of twenty horses, if used with considerable expansion. The steam is condensed in a number of pipes of small diameter, which are exposed to the strong current of air produced by the flight: this mode of condensation has been found remarkably effective. All unnecessary weight of parts has been avoided, and indeed no part has been retained whose services are not essential. The result is, that a twenty-horse engine is kept in efficient action with but twenty gallons of water, and its entire weight is but about 600lbs.

The weight of the whole machine, and its load, is estimated at 3000 lbs: the area of the sustaining surfaces will be about 4500

square feet. The load will, therefore, be about two-thirds of a pound to each square foot, which is less by one-third than that of many birds.

The most important question which remains to be decided refers to the competency of the steam-engine; and here unhappily mechanical science and experimental facts alike fail to give us the needful information.

As far as probabilities can be collected from observations on the flight of birds, they warrant a strong expectation of Mr. Henson's success. If, however, his engine should be found to need reinforcement, it is said there are available inventions recently matured, whose combined application will much more than double its power. Nor can it be doubted that, cleared as the subject now is of its mysteries and chief difficulties, the attention of our engineers will be strongly drawn to the subject, and the inventive energies of this mechanical age speedily bring the machine to perfection.

One of the most remarkable as it is one of the most cheering considerations connected with this subject is the fact, that those improvements in locomotion are ever first committed by Providence to that part of the human family which is at the time best fitted to use them for the general benefit;-best fitted, we mean, not so much by the extent and firmness of their political relations, or the energy of their enterprise, or the magnitude of their capital, though these are far from indifferent, as by the moral temperament which they will bring to their entrusted employment. Savages, who without restraint of conscience might desolate with grim delight the enlarged circle put within their reach are not invested with these new powers! nor even when the unwonted device is placed before their eyes have they the means, the energy or the intellect to use it with effect at all to be compared with that of its employment with more advanced communities; invention and its results seem nearly dormant, except for the purposes to which it can be applied by the most enlightened portions of the race. And if so in all past time, may we hope to discover in the circumstances attending this new and unparalleled enterprise, traces of the same great design, and may we not easily suppose that so long as the new art, should it come into practical use, shall require the appliances of capital, of cultivated skill, of tried integrity, and of the most exact and elaborate science, so long it will be mainly in the hands of that section of the wide earth's inhabitants who are most likely to use its astounding capabilities in the spirit of justice and good-will to all.