Sir,-In the book-case of a deceased friend I found, several years ago, a curious pamphlet, which I have every reason to suppose is scarce, or perhaps was printed for the author's friends alone. In it you will find a sound and rational, and scientific theory on aërostation--founded on a true deduction from cause to effeet; and as aërostation once more forms the subject of your columns in the instance of the use of balloons during the revolutionary war with France, I am anxious to direct the attention of ingenious mechanics and others to the only exact theory which I have seen promulgated. None of the encyclopedias, nor other works of ancient and modern science, can give any instance of Mr. Walker's theory having been tried, or even fairly worked upons and I am inclined to believe, that this pamphlet will furnish most of your readers with some information on the subject, not to be gained elsewhere.

When I made some inquiries respecting the author, on my passing through Hull a few years ago, he was not to be found, I heard, however, that in addition to the plan suggested in his pamphlet, he had proposed to fix a triangular parachute on a light mast over the aëronaut; supplying at once safety in case of accident, and suspensive power in addition to that gained by the movement of the wings. I hand you a rude sketch of the position of the parachute, (See Fig, at the top of p. 131.) The subject of the pamphlet you will probably abridge, or copy verbatim.

Your's,

H. BELL.

self to ascend and soar through the air with the facility of a bird Illustrated with plates, by Thomas Walker, portrait painter, Hull.” Printed at Hull, 1810, pp. 67.

Mr. Walker commences his treatise, by pronouncing all the former methods of flying which have been mere whims and proposed, to be " contrivances, utterly destitute of the true nature and science of flying;' and not, like the one now to be submitted to the reader, founded on a

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study of the works of nature." This study, he says, "has been to me, during the greatest part of my life, a source of amusement and inexpressible delight. The natural history of birds has particularly occupied my attention, and that enviable faculty which they possess of flying, has greatly excited my curiosity, and led me to that study by which I have obtained a true knowledge of the mechanical principles by which they fly; a knowledge which, I do not hesitate to declare, has hitherto remained undiscovered, although it has been the object of the study and contemplation of many of the most eminent My philosophers of past ages. invention," he adds, "is founded entirely upon the principles of nature; and although these principles are as old as the creation, they have never until now been properly attended to."

The author next proceeds to give the following brief, and rather imperfect, account of the schemes previously proposed by others for navigating the air.

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In almost every nation where arts and sciences have flourished, person's have manifested a wish to discover the art of flying. In Roine and in Paris particularly, different persons, and in ages remote from each other, have tried experiments with wings formed of various materials, which have been fastened to their arms; but none of them succeeded, there not being strength sufficient in a man's arms to enable him to fly with detached wings fastened to him, leaving the whole weight of his body unsupported.

Friar Bacon, who lived nearly five centuries ago, wrote upon the subject, and he affirms that the art of flying is possible; and many others have been of opinion, that by means of artificial wings affixed to the arms or legs, a man might fly as well as a bird.

The philosophers of the reign of King Charles the Second, were much engaged with this art. The famous Bishop Wilkin, who, in 1672, published a treatise upon fly ing, was so confident of its practica bility, that he says, he does not question but that in future ages it will become as common to hear a man call for his wings, when going a journey, as it is now to call for his boots and spurs.

"In the year 1709, as we gather from a letter published in France in 1784, a Portuguese, Friar de Gusman, applied to the king to encou rage him in the invention of a flying machine. The principle upon which it was constructed, if indeed it had any principle, seems to have been that of a paper kite; the machine was in the form of a bird, and contained several tubes through which the wind was to pass in order to fill a certain sail, which was to elevate it; and when the wind was deficient, the same was to be effected by means of bellows concealed within the body of the machine. The ascent was also to be promoted by the electric atstraction of pieces of amber placed in the top, and by two spheres in

"These silly inventions shew the very low state of science at that time in Portugal, especially as the king, in order to encourage him in his further experiments in such an useful invention, granted him the first vacant place in his college of Barcelos, or Santerim, with the first professorship in the university of Coimbra, and an annual pension of 600,000 reis, during his life. Of this De Gusman it is also related, that, in the year 1736, he made a wicker basket of about seven or eight feet diameter, and covered it with paper, which raised itself about 200 feet in the air, and the effect was generally attributed to witchcraft.

"Mr. Willoughby, after observing that the pectoral muscles of a man, in proportion to his weight, are many degrees too weak for flying, recommends to him who would at, tempt the art with the desire of success, to contrive and adapt his wings in such a manner, that he may work them with his legs and not with his arms, because the mus cles of the legs are much stronger.

The celebrated Lord Bacon wrote on the subject of flying, and believed it practicable; but it seems that he could no more direct how it was to be done than any other who had written before him on the same subject."

Mr. Walker then adverts to his own notions on the subject

"It is now almost twenty years since I was first led to think, by the study of birds and their means of flying, that if an artificial machine were formed with wings, in exact imitation of the mechanism of one of those beautiful living machines, and applied in the very same way upon the air, there could be no doubt of its being made to fly; for it is an axiom in philosophy, that the same cause will ever produce the same effect.

"It is easy to demonstrate, that a bird is no more able to fly than a man without the mechanical effect of wings; therefore, when a man is furnished with a pair of wings large enough, and can apply them in the same manner as a bird does, and with sufficient power, there can be no reason to doubt of a man being able to fly as well as a bird. The machine which I have planned is as close a copy of the natural mechanism of a bird as artificial means will admit of; and when my readers are made thoroughly acquainted with both the natural and artificial means of flying, I flatter myself they will then be willing to acknowledge that my scheme is a very rational one, highly calculated to insure success in the accomplishment of the art of flying, one of the most extraordinary and desirable arts with which we can be acquainted."..

In order to exemplify more particularly the powers of flight possessed by birds, Mr. Walker introduces the following interesting description of the condor :

The ostrich, in the torrid regions of Africa; the emu, in the extensive plains of Paraguay, in South America, which standing erect, is about seven feet high, its legs are three feet long, its thighs are nearly as thick as the thighs of a man; it runs so swift that the fleetest dogs are foiled by it; the cassowary and the dodo, in the Molucca Islands; and the penguins, in the Straits of Magellan and the South Sea Islands; all these birds are as utterly incapable of flying as a man, none of them being provided with wings for that purpose.

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"The condor is a native of America, and hitherto naturalists have been divided whether to refer it to the species of the eagle, or to that of the vulture. Its great strength and activity, seem to give it a claim to rank among the former, whilst the baldness of its head and neck is thought to degrade it to a rank amongst the latter. It is, however, fully sufficient for our plan to describe its manners, form, weight, expansion and power; we will therefore leave to momenclators to decide upon its class. If size, for it is by much the largest bird that flies, and strength, combined with rapidity of flight, and rapacity, deserve preeminence, then no bird can be put in competition with it; for the condor possesses, in a higher degree than the eagle, all the qualities that render it formidable, not only to the feathered tribe, but to beasts, and even to man himself.

"Acosta, Garcilasso, and Desmarchais assert, that it measures eighteen feet across the wings when expanded: its beak is so strong as to pierce the body of a cow; and it is positively asserted that two of them are capable of devouring that animal. They do not even abstain from attacking man himself; but fortunately there are but few of the species. The Indians say that they will carry off a deer, or a young calf in their talons, as an eagle would a hare, or a rabbit ; that their sight is piercing, and their manners terrific. According to modern authors they only come down to the

sea-coast at certain seasons, particularly when it is supposed their prey fails them upon the land; that they then feed upon dead fish, and such other nutritious substances as the sea throws upon shore.

"Condamine says, he has frequently seen them in several parts of the mountains of Quito, and has observed them hovering over a flock of sheep; and he thinks they would, at one particular time, have attempted to carry some of them off, had they not been scared away by the shepherds. Labat says that this bird has been described to him, by those who have seen it, as having a body as large as a sheep, and that its flesh is as tough and disagreeable as carrion. The Spaniards, residing in that country, dread its depredations, there having been many instances of its carrying off children.

Mr. Strong, the master of a ship, relatės, that, as he was sailing along the coast of Chili, in the thirty-third degree of south latitude, he observed a bird sitting upon a high cliff near the shore, which one of the ship's company shot with a leaden bullet and killed. They were greatly surprised when they beheld its magnitude; for when the wings were extended they measured thirteen feet from one tip to the other, one of the quill feathers was two feet four inches and three quarters in length, and an inch and a half in circumference.

"Mons. Feuillée, whose description alone is accurate, has given a still more circumstantial account of this amazing bird.

"In a valley of Illo, in Peru, says he, I discovered a condor perched on a high rock before me; I approached within gun-shot, and fired; but, as my piece was only charged with swan-shot, the lead was not heavy enough to bring the bird down. I perceived, however, by its manner of flying, that it was wounded; and it was with a good deal of difficulty that it flew to another rock, about five hundred yards distant on the sea-shore. I therefore charged again with ball, and hit the bird under the throat, which made it mine. I accordingly ran up to seize; but

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even in death it was terrible, and defended itself upon its back, with its claws extended against me, so that I scarcely knew how to lay hold of it. Had it not been mortally wounded, I should have found it no easy matter to take it; but I at last dragged it down from the rock, and, with the assistance of one of the seamen, I carried it to my tent, to make a coloured drawing of it. The wings of this bird, which I measured very exactly, were twelve feet three inches (English) from tip to tip. The great feathers, which were of a beattiful shining black, were two feet four inches long. The thickness of the beak was proportionable to the rest of the body; the length about four inches; the point hooked downwards, and white at its extremity; and the other part was of a jet black. The thigh bones were ten inches long, the legs five inches, the toes and claws were in proportion; and the legs were covered with black scales. The little nourishment which these birds find on the coast, except when a tempest throws up some great fish, obliges the condor to continue there but a short time. They usually come to the coast at the approach of evening, stay there all night, and fly back in the morning.'

(To be Continued.)

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follows that when they arrange themselves in conformity with the equatorial currents of the earth, that are circulating east and west, that axis and the whole needle will point to the north and south-as we find they actually do.

The nature of this influence is more clearly discernible when it is exhibited in its simplest form, on a single circular current, which, as we have seen, may be regarded as the element of a magnet. A conducting wire, bent into the form of a circle, when free to move, always assumes, by the electro-magnetic action of the earth, a position in a plane descending to the south, intersecting the horizon in a line passing east and west, and inclined to it at an angle which is the complement to the dip; that is, in a plane which is perpendicular to the magnetic meridian. Its northern and southern polarities are equally real with that of a magnetic needle; but appertain to an imaginary axis passing through the centre of the circle and perpendicular to its plane. The direction of the currents, on its south side, or that nearest the equator, is similar to those in the earth's equator; that is, from east to west, the positive current being always understood as defining the direction.

The same phenomena are observed if the course of the moveable wire be that of a parallelogram, or, indeed, any plane figure which returns into itself, as well as if it were a circle.

The action of the earth on spiral conducting wires, is precisely similar in kind to that on single circles, but it is more powerful in degree. Helices, in like manner, are found to obey the terrestrial influence just as magnets do when placed in similar circumstances, as to freedom of motion, provided the electrical currents which they convey are of sufficient intensity. In fact, every experiment that has been tried has served but to confirm the correctness of Ampère's theory of magnetism,

Ampère, however, so far from supposing that the successive action of the solar rays on the equatorial regions, is the sole cause of the electric currents that circulate there, internal changes taking place in the earth itself, must, he conceives, concur in producing them. The diurnal variations may, however, be fairly enough attributed to the alternate changes of temperature occurring in different parts of the torrid, and even of the temperate zones.

Numerous facts have induced Ampere to conclude, that the circulation of

electric currents peculiar to magnets, takes place round each particle of the magnetic body. He is also of opinion, that these currents pre-existed in the bodies susceptible of magnetism, before this property was imparted to them; but, as they were moving in every variety of direction, they neutralized each other, and could produce no external effect. It is only when a determinate direction has been given to them, either by another magnet, or by a voltaic current, that they become capable of exerting any magnetic action.

Ampère has farther proved, by experiment, that a powerful electrical current has a tendency to excite similar currents in neighbouring bodies-a brass ring for example-not generally susceptible of magnetism.

The Reviewer remarks, in conclusion, that the theory of Ampère has the advantage of explaining many anomalous appearances in the induction of magnetism, much more readily than any other; as, for instance, the consequences observed on the fracture of a magnet-the opposite polarities being supposed to arise from the same currents, and only to take place on the different sides of these currents; and the want of success in all the attempts which have been made to obtain any of the ordinary electrical or galvanic effects, from whatever form or combination of magnets has yet been devised.

It is confessed, however, that much remains to be done before the theory of Ampère can be regarded as satisfactorily established. The new discoveries which are continually making in electro-magnetic science, will, however, be subjected to a severe trial, and must soon either give it a decisive confirmation, or produce its complete overthrow. Many curious facts, lately brought to light, by Arago, Barlow, Christie, Babbage, and Herschel, relative to the magnetic effects induced on iron, and the magnetism manifested by other metals during their rotation, are yet to be explained consistently with Ampère's theory. The recent discovery, too, of the influence of the solar rays, in inducing permanent magnetism in iron, under certain circumstances, a fact announced by Morachini, and now completely established by the ingenious and successful experiments of Mrs. Somerville,* indicates the existence of some hitherto

*Philosophical Transactions for 1826, p. 132. See, also, Mechanics' Magazine, vol. v. p. 383.