the metal is capable. It is, however, more usual to employ the process called the double touch: placing two magnets, with their opposite poles near to each other, or the opposite poles of a single magnet, bent into the form of a horse-shoe, in contact with the middle of the bar: the opposite actions of these two poles then conspire in their effort to displace the magnetic fluid, and the magnets having been drawn backwards and forwards repeatedly, an equal number of times to and from each end of the bar, with a considerable pressure, they are at last withdrawn in the middle, in order to keep the poles at equal distances.

Iron filings, or the scoriæ from a smith's forge, when finely levigated, and formed into a paste with linseed oil, are also capable of being made collectively magnetic. A bar of steel, placed red-hot between two magnets, and suddenly quenched by cold water, be comes in some degree magnetic, but not so powerfully as it may be rendered by other means. For preserving magnets, it is usual to place their poles in contact with the opposite poles of other magnets, or with pieces of soft iron, which, in consequence of their own induced magnetism, tend to favour the accumulation of the magnetic power in a greater quantity than the metal can retain after they are removed. Hence the ancients imagined that the magnet fed on

iron.

A single magnet may be made of two bars of steel, with their ends pressed into close contact; and it might be expected that when these bars are separated, or when a common magnet has been divided in the middle, the portions should possess the properties of the respective poles only. But in fact the ends which have been in contact are found to acquire the properties of the poles opposite to those of their respective pieces, and a certain point in each piece is neutral, which is at first nearer to the newly formed pole than to the other end, but is removed by degrees to a more central situation. In this case we must suppose, contrarily to the general principles of the theory, that the magnetic fluid has actually escaped by degrees from one of the pieces, and has been received from the atmosphere by the other.

There is no reason to imagine any immediate connexion between magnetism and electricity, except that electricity affects the conducting powers of iron or steel for magnetism, in the same manner as heat or agitation. In some cases a blow, an increase of temperature, or a shock of electricity, may expedite a little the acquisition

of polarity; but more commonly any one of these causes impairs the magnetic power. Professor Robinson found, that when a good magnet was struck for three quarters of an hour, and allowed in the mean time to ring, its efficacy was destroyed; although the same operation had little effect when the ringing was impeded; so that the continued exertion of the cohesive and repulsive powers appears to favour the transmission of the magnetic as well as of the electric fluid. The internal agitation, produced in bending a magnetic wire round a cylinder, also destroys its polarity, and the operation of a file has the same effect. Mr. Cavallo has found that brass becomes in general much more capable of being attracted when it has been hammered, even between two flints; and that this property is again diminished by fire: in this case it may be conjectured that hammering increases the conducting power of the iron contained in the brass, and thus renders it more susceptible of magnetic action. Mr. Cavallo also observed that a magnetic needle was more powerfully attracted by iron filings during their solution in acids, especially in the sulphuric acid, than either before or after the operation: others have not always succeeded in the experi ment; but there is nothing improbable in the circumstance, and there may have been some actual difference in the results, dependent on causes too minute for observation. In subjects so little understood as the theory of magnetism, we are obliged to admit some paradoxical propositions, which are only surprising on account of the imperfect state of our knowledge. Yet, little as we can understand the intimate nature of magnetical actions, they exhibit to us a number of extremely amusing, as well as interesting, phænomena; and the principles of crystallization, and even of vital growth and reproduction, are no where so closely imitated, as in the arrangement of the small particles of iron in the neighbourhood of a magnet, and in the production of a multitude of complete magnets, from the influence of a parent of the same kind.

[Young's Natural Philosophy.

[63]

CHAP. IV.

AEROSTATION, INCLUDING THE PRINCIPLES, HISTORY, AND MANAGEMENT OF BALLOONS.

SECTION I.

Principles of Aërostation.

THE fundamental principles of this art have been long and generally known, as well as the speculations on the theory of it; but the successful application of them to practice seems to be altogether a modern discovery. These principles chiefly respect the weight or pressure, and elasticity of the air, with its specific gravity, and that of the other bodies to be raised or floated in it; the particular detail of which principles, however, we have not space to enlarge upon. Suffice it therefore, for the present, to observe, that any body which is specifically, or bulk for bulk, lighter than the atmosphere, or air encompassing the earth, will be buoyed up by it, and ascend, like as wood, or a cork, or a blown bladder, ascends in water. And thus the body would continue to ascend to the top of the atmosphere, if the air were every where of the same density as at the surface of the earth. But as the air is compressible and elastic, its density decreases continually in ascending, on ac. count of the diminished pressure of the superincumbent air, at the higher elevations above the earth; and therefore the body will as cend only to such a height where the air is of the same specific gravity with itself; where the body will float, and move along with the wind or current of air, which it may meet with at that height. This body then is an aërostatic machine, of whatever form or na ture it may be. And an air-balloon is a body of this kind, the whole mass of which, including its covering and contents, and the weights annexed to it, is of less weight than the same bulk of air in which it rises. We know of no solid bodies, however, that are light enough thus to ascend and float in the atmosphere; and therefore recourse must be had to some fluid or aeriform substance. Ainong these, that which is called inflammable air, the hydrogen gas of the new nomenclature, is the most proper of any that have hitherto been

discovered. It is very elastic, and from six to ten or eleven times lighter than common air; and consequently this compound mass will rise in the atmosphere, and continue to ascend till it attain a height at which the atmosphere is of the same specific gravity as itself; where it will remain or float with the current of air, as long as the inflammable air does not escape through the pores of its covering. And this is an inflammable air-balloon. Another way is to make use of common air, rendered lighter by warming it, instead of the inflammable air. Heat, it is well known, rarefies and expands common air, and consequently lessens its specific gravity; and the diminution of its weight is proportional to the heat applied. If therefore the air, inclosed in any kind of a bag or covering, be heated, and consequently dilated, to such a degree, that the excess of the weight of an equal bulk of common air, above the weight of the heated air, he greater than the weight of the covering and its appendages, the whole compound inass will asceud in the atmosphere, till, by the diminished density of the surrounding air, the whole becomes of the same specific gravity with the air in which it floats; where it will remain, till, by the cooling and condensation of the included air, it shall gradually contract and descend again, unless the heat is renewed or kept up. And such is a heated air-balloon, otherwise called a Montgolfier, from its inventor. Now it has been discovered, by various experiments, that one degree of heat, according to the scale of Fahrenheit's thermometer, expands the air about one five-hundreth part; and, therefore, that it will require about 500°, or nearer 484° of heat, to expand the air to just double its bulk: which is a degree of heat far above what it is practicable to give it on such occasions. And, therefore, in this respect, common air heated is much inferior to inflammable air, in point of levity and usefulness for aerostatic machines. Upon such principles then depends the construction of the two sorts of air-balloons. But before treating of this branch more particularly, it will be proper to give a short historical account of this late-discovered art.

VARIOUS schemes for rising in the air, and passing through it, have been devised and attempted, both by the ancients and moderus, and that upon different principles, and with various success.

Of these, some attempts have been upon mechanical principles, or by virtue of the powers of mechanism: and such are conceived to be the instances related of the flying pigeon made by Archytas; the flying eagle and fly by Regiomontanus, and various others. Again, other projects have been formed for attaching wings to some part of the body, which were to be moved either by the hands or feet, by the help of mechanical powers; so that striking the air with them, after the manner of the wings of a bird, the person might raise him. self in the air, and transport himself through it, in imitation of that animal. The romances of almost every nation have recorded instances of persons being carried through the air, both by the agency of spirits and mechanical inventions; but till the time of the celebrated Lord Bacon, no rational principle appears ever to have been thought of by which this might be accomplished. Friar Bacon, in. deed, had written upon the subject; and many had supposed, that, by means of artificial wings, a man might fly as well as a bird: but these opinions were refuted by Borelli in his treatise De Motu Ani. malium, where, from a comparison between the power of the mus cles which move the wings of a bird, and those which move the arms of a man, he demonstrates that the latter are utterly insufficient to strike the air with such force as to raise him from the ground. In the year 1672, Bishop Wilkins published his "Discovery of the New World," in which he certainly seems to have conceived the idea of raising bodies into the atmosphere by filling them with rarefied air. This, however, he did not by any means pursue; but rested his hopes upon mechanical motions, to be accomplished by human strength, or by springs, &c. which have been proved inca. pable of answering any useful purpose. The jesuit, Francis Lana, cotemporary with Bishop Wilkins, proposed to exhaust hollow balls of metal of their air, and by that means occasion them to ascend. But though the theory was unexceptionable, the means were certainly insufficient to the end: for a vessel of copper, made sufficiently thin to float in the atmosphere, would be utterly unable to resist the external pressure, which being demonstrated, no attempt was made upon that principle. So that we may reckon nothing to have been particularly concerted towards aerostation, till the experiment of one Gusman, a Portuguese friar, who is reported early in the last century to have launched a paper bag into the air; which, however, soon fell, after attaining the height of 100 feet. Soon after Mr. Cavendish's discovery of the specific gravity of inflammable air, it occurred

VOL. VI.