As it is neither those which are of the smallest specific gravity, or clothed with the lightest down, that are most capable of flight, it follows, that flying is accomplished merely by a mechanical operation ; more particularly on considering how much it is diversified among the feather ed tribes ; one bird is continually soaring aloft at the great est altitudes, another skims the earth without intermission, while a third only displays its wings occasionally, and is as if propelled by another agent when they open or close. Bishop Wilkins, that ingenious philosopher, whose works are too little studied at the present day, judiciously ob serves : " We see a great difference betwixt the several quantities of such bodies as are commonly upheld by the air : not only little gnats and flies, but also the eagle and fowls of vaster magnitude." Many insects, even some in this country, exceed the diminutive size of the humming bird, which is but an eighth of an ounce in weight. It is almost constantly on the wing, apparently sipping with its tender bill from the nectarium of the flowers. An enor mous bird, the condor of South America, is calculated to be 8162 times heavier : " What an amazing disproportion of weight !" exclaims a modern author ; " yet, by the same mechanical use of its wings, the condor can overcome the specific gravity of its body with as much ease as the little humming bird. But this is not all ; we are informed that this immense bird possesses a power in its wings so far ex ceeding what is necessary for its own conveyance through the air, that it can take up and fly away with a whole sheep in its talons, with as much ease as an eagle would carry off, in the same manner, a hare or a rabbit. This we may rea dily give credit to, from the known fact of our little kes tril, and the sparrow hawk, flying oil with a partridge, which is nearly three times the weight of either of these rapacious little birds." A calculation is next made of the combined weight of the condor and its prey, which amounts to 20,405 times the weight of the humming bird, to be borne through the air. Probably the author would have found similar illustrations among the more rapacious wing ed insects, whose wings are less adapted for it. But he proceeds with another comparison, to prove that the length of the wings of birds is not augmented in proportion to the increased weight of their bodies, whence he infers the pos sibility of constructing a machine with which a man should be enabled to fly. " The condor carries ten stone with wings of 12 feet expansion from tip to tip. The humming bird carries one drachm with three inches expansion ; the common wren is three times as heavy as the humming bird, and has but one inch more of wing ; a pigeon weighs 16 ounces, which is 256 times as heavy as it is, and has only ten times more expansion of wing ; the goat-sucker is 40 times as heavy, and has seven times the length of wing. Therefore, as a man weighing ten stone, and a machine to bear him two, will only exceed the weight of the condor and kW prey by one fifth part, and as the wings of the con dor are about 12 feet ; suppose we make a pair of wings of silk, one fifth longer than they are, which will be about fourteen and a half, I am thoroughly persuaded they will be found amply sufficient, as they will far exceed the pro gressive increase of birds wings." Authors have even speculated on the fashion and sub stance of the wings, and in general have concluded that they should be analogous to those of birds. Bishop Wil kins, after observing that if there be any such artificial contrivances that can fly in the air, then it will clearly fol low, that it is possible also for a man to fly himself ; and he recommends the wings to be formed of feathers, like those employed by Daedalus, or else of one uninterrupted sub stance like those of bats. " But now because the arms ex tended are but weak and easily wearied, therefore the mo tions by them are like to be but short and slow, answera ble, it may be, to the flight of such domestic fowl as are most conversant on the ground ; and therefore much more would the arm of a man, as being not naturally designed to such a motion. It were, therefore, worth the enquiry, to consider whether this might not be more probably ef fected by the labour of the feet, which are naturally more strong and indefatigable. In which contrivance the wings should come down from the shoulders on each side, as in the other ; but the motion of them should be from the legs being thrust out and drawn in again, one after another, so as each leg should move both wings, by which means a man should as ,it were walk or climb up into the air." In all later proposals, however, the idea of flying by exerting the animal powers alone on wings has been abandoned ; though the utmost confidence of its success continues still to be entertained, and, as we shall immediately see, it has actually been pUt in practice. The close imitation of na ture, also, under the modification required by the difference of materials, is invariably to be preserved, Among the most recent authors on this subject, may be named Sir George Cayley, who endeavours to chew that there is nothing adverse to the soundest reasoning in ex pecting to overcome the difficulties which men experience in elevating themselves in the air. Ile shews that flight is purely mechanical, for by a simple experiment in dispo sing four quill-feathers at right angles to each end of a rod, and presenting an oblique surface to the air, this apparatus will of itself rise, when a rotatory motion is produced by the relaxation of a spring untwining a cold connected with it, and encircling the tod. " The flight of a strong man by great muscular strength," he observes, "though a curious and interesting circumstance, in as much as it will proba bly be the first means of ascertaining this power, and sup plying the basis whereon to improve it, would be of little use. I feel perfectly confident, however, that this noble art will soon be brought home to man's general Conveni ence ; and that we shall be able to transport ourselves and families, and their goods and chattels, more securely by air than by water, and with a velocity of from 20 to 100 miles per hour." Bishop Wilkins, indeed, was long ago so confident of success, that he anticipated the period W hen a person should as readily call for his wings to make a journey, as he then did for his boots and his horse. Si: George Cayley continues : " To produce this effect, it is only necessary to have a first mover, which will generate more power in a given time, in proportion to its weight, than the animal system of muscles." Ile seems to infer, that he has made experiments on a considerable scale ; but we are not acquainted with their nature or result ; and we are precluded from discussing his theories, by the neces sity of referring to what seems more decidedly practical.
Just about the same time, Mr Walker of Hull, whose sentiments we have already quoted, directly proposes a machine whereby flying shall be accomplished ; and he maintains that he is the first person who has discovered the real theory of the flight of birds. On considering the structure of birds, he maintains, that by means of a pair of wings and a tail duly expanded in a perfectly passive state, arid aloft in the air, without any muscular motion, a bird procures a suspending power, which counteracts the spe cific gravity of its body, and prevents its being precipitated to the ground. But this is perhaps assuming too much ; for it is probable, that although we are not sensible of the action of the wings in birds apparently suspended motion less in the air, they would fall to the earth without it. With
respect to the quill-feathers, which are here the prime agents, he observes, that, as they were intended to swim in a fluid so light and subtile as the air, it was necessary they should consist of the lightest materials imaginable ; that being intended to strike upon the air with great power and effect, it was requisite that the shafts should possess much strength with elasticity. " It was expedient, too, that the quill-feathers should shut and open, to let the upper air pass through the wings, to facilitate their ascent when they arc struck upwards. It was also necessary that they should all shut close together, forming each wing into a complete surface or web, when they are by the muscular power of the bird forced clown, in order to give a more secure hold upon the air below, and by their means keep the bird tip. Now, if we do but examine the quill-feathers, we shall find in the shafts astonishing strength with elasticity, and very little specific gravity indeed. The webs are broader on one side of the shafts than the other, which causes them to open as the wings move up, and to shut as they come down, exactly answering the purposes I have already mentioned." With regard to the operation and effect of the wings and tail in an active state, it appears that flight is attained from the resistance of the air or percussion. " When a bird, by the power of its pectoral and deltoid muscles, puts its wings into action, and strikes them downwards in a per fectly vertical direction upon the air below, that air, being compressed by the stroke, makes a resistance by its elastic power against the under side of me wings, in proportion to the rapidity of the stroke and the dimensions of the wings, and forces the bird upwards: at the same time, the back edges of the wings being more weak and elastic than the fore edges, they give way to the resisting power of the compressed air which rushes upwards against the same back edges, acting against them with its elastic power, and thereby causes a projectile force which impels the bird forward. And thus we see that by one act of the wings, the bird produces both buoyancy and progression. When the tail is forced upwards, and the wings are in action, the bird ascends ; and forced downwards, it consequently de scends. But the most important use of the tail is to sup port the posterior weight of the bird, and to prevent the vacillation of the whole." Fortified by these principles, Ali' Walker proposes an apparatus, whereby, from the action of wings, flying shall he attained ; and this is to consist of a case formed of light materials, provided with wings of the requisite dimensions, to be put in action by a man sitting, and as if rowing a boat. The wings are recommended to be each about eight feet long when horizontally expanded ; and fastened upon the top edge of the car, with two joints each, so as to admit a vertical motion to the wings, which motion may be effected by a man sitting and working an upright lever in the middle of the car. A tail of seven or eight feet long, and the same breadth at its extremity, must be fixed to the hinder part of the car, and spread out flat to the horizon, in the same manner as we see the tail of birds." Considerable atten tion must be paid to the structure of the wings and tail, a point that has never escaped speculators on this subject ; and Mr Walker proposes that the shafts of each are to consist of six slips of thin whale-bone, dressed, and taper ing to a point ; then wrapped together from end to end in a round form with small twine, and filled with cork along the inside. They are next to be covered with silk, very compactly woven, and as impervious to the air as possible. This is to be laid on in separate broad slips, and should open to admit the passage of the air as the wings move up, and close together again as they come doWn, operating in the same manner as the quill-feathers in the wings of birds. But such a peculiarity may be found unnecessary on expe riment, because, as the author observes, " we see flying squirrels, bats, butterflies, flying fish, Re. with wings form ed of compact membranes, all flying exceedingly well." It is essential that the car be externally covered with silk or very thin leather, which must be united to the base of the wings along each side of the car, to prevent, as much as possible, the air from escaping any where but from the back edges of the wings. Should that be neglected, when the air is compressed by the wings being struck down wards, it will rush upwards through the car, and thereby fail of giving that resistance against the under side of the wings, which is necessary to give buoyancy and progres sion. The whole is to be considered " as a large artificial bird, and the man placed in the inside as the vital or mov ing power." Such is the flying machine by which Ti Walker expects to accomplish a journey to the ethereal re gions. In studying i'.s operation, he particularly directs our attention to the propulsion occasioned by the reaction of the air against the under side and back edges of the wings. " When the air is struck by the wings, the effect of its reaction against their under side and back edges is si milar to that which is caused by the wind blowing with suf ficient force against a mill sail when it rushes off on one side, and impels the sail to move, with this difference only, that the sail, being fastened at one end of an axis, is made to revolve ; whilst the bird, being at full liberty in the air, is caused. by the expansive power of the air acting with a resisting force against the back edges of the wings, to glide forward hi a right line." We shall abstain from following the author in his details respecting the manner in which the aerial navigator is to commence his flight ; how he is to clear the tops of houses, trees, and hills, with safety ; and how he is to guide himself through the subtile fluids floating high above the earth : but he is no less confident than Sir George Cayley, of the speed of his journey ; and in answer to :my objections against the utility of flying, lie says, " I hope it will be granted, that flying will be of great use, if by such means we can have our letters, newspapers, &c. conveyed to any part of the kingdom, at the rate of 40 or 50 miles in an hour: or if that numerous class of mer cantile agents, riders, henceforth he enabled to glide through the air with great expedition in flying machines : or if a man by such means can take a rope to any mariners in distress along the sea-coast, and thereby be the happy instrument of saving their lives: and if the circumnaviga tor be able to quit his ship, fly, and explore the interior parts of a new-discovered island, free from the annoyance and hostilities of its rude inhabitants." The expectations of these authors would certainly not be disappointed, could the flight of man, if it be attainable, approach the speed of the feathered tribes. Birds dart through the air ; the eye can scarcely follow them ; and the largest can almost instantaneously seek those points and altitudes, where they are lost to human vision. Some conjectures have been of the rate of this extraor dinary velocity. Sir George Cayley computes the flight of the common crow at 34} feet in a second, or above 25 miles an hour. Cartwright calculated the flight of wild ducks on the coast of Labrador, et 90 miles an hour. Spall anzani, by actual experiment, found that a swallow flew 20 miles in 13 minutes, or at the rate of 92 miles an hour ; but he conceives, that the swift or martin can traverse no less than 60 miles in 15 minutes.