cal occupations. From the name of that place, the fociety derives its appellation of Arcueil. Befides La Place, who appears rather as a patron and counsellor, the members confift of the younger Berthollet, Biot, Gay-Luffac, Humboldt, Thenard, Decandolle, and Collet-Descoftils. At their meetings, the latest fcientific journals are confulted, philofophical papers are read and difcuffed, and new experiments are propofed, repeated, or fet on foot. The advantages of fuch a plan are most obvious. Miftakes may be detected, errors avoided, and important lights ftruck out by the collifion of ideas. In the actual ftate of science, no experiments are truly valuable, but thofe which have been performed with the moft fcrupulous precifion. The art of experimenting itself has now become fo refined, and attended with fuch vaft expenfe, as often to lye beyond the reach of individual exertion. are, therefore, inclined to augur favourably of a fociety of this nature, which defcends to guide and aflift the details of inquiry. If our expectations have not been fully anfwered, we yet difcern the germs of more important communications; and truft that fimilar affociations, furnished with more ample means, will foon be formed at home.-We confine our felections to thofe papers which appear the most worthy of notice. 1. Observations on the Intensity and Inclination of the Magnetic Force, made in France, Switzerland, Italy, and Germany. By Mellieurs Humboldt and Gay-Luffac. The laws of magnetic action were first difcovered by our ingenious countryman Dr Gilbert of Colchefter. That original phi lofopher, who, prior to the writings of Bacon, understood and fuccefsfully pursued the method of induction, reduced all the phenomena of magnetism to four general facts: 1. That, of two magnets, the fimilar poles repel and the diffimilar attract each other; 2. That these attractive and repulfive forces are increased by proximity; 3. That a magnet, by mere appofition, has a power of inducing magnetifm on a piece of iron or steel, and which is more or leis durable according to the hardnefs or softness of the material affected; and, 4. That the mafs of the earth itself contains an immenfe magnet, poffefling those diftinctive properties. From fuch data, it was eafy to explain the more obvious effects of magnetifin. A magnet attracts a bit of iron, because the iron, for the time at leaft, becomes likewife a magnet, endued with polarity; and a magnetic bar, freely fufpended, turns towards the north, from the influence of the internal magnet of our globe. When Gilbert published his theory, the needle at London ftood very nearly in the line of the meridian; but it was afterwards obferved to deviate gradually to the weft, with a declination that has, for about about two centuries, been continually increasing. The rate of this digreffion, indeed, has of late years fenfibly relaxed; which affords a prefumption, that, perhaps in the course of another century, the needle will have finifhed its period of aberration, and may return again by the fame fteps. The variation of the mariner's compafs differs, however, in each particular place: fometimes it is stationary, but generally it is either advancing or retreating, and that with a progrefs unequal, and ever fubject to change. Such a fyftem of perpetual mutation is in the highest degree perplexing, and forms one of the greateft obftacles to the practice and improvement of navigation. But all this intricacy and feeming irregularity may refult from the combination of a few very fimple changes. What could, at firft fight, appear more involved than the motions of the heavenly bodies? And yet, when the feparate elements. are developed, how fimple and harmonious the whole becomes ! There is the ftrongest reafon, therefore, to conclude, that the complicated aberrations of the needle may proceed from certain regular changes in the pofition of the poles of the terrestrial magnet. The great desideratum is now to ascertain the nature of those changes. For that purpose, it is neceffary to determine accurately, at diftant points on the earth's furface, the direction and intensity of the magnetic action. Hitherto, the direction only has been obferved, by help of the compafs and dipping needle; and even thefe obfervations, from the imperfection of the latter inftrument, are in many cafes doubtful or defective. To difcover the relative intenfities of magnetic action at different places, would lead more immediately to the folution of the problem. In the mutual action of magnets, four separate forces are exerted. The nearer pole of the one attracts and repels the poles of the other; while its farther pole, reversing the order, repels and attracts the same poles. These blended forces are capable of producing two distinct effects: 1. The one magnet may turn about a fixed axis, from the conjoined action of all the forces, and consequently with a power equal to their aggregate sum; and 2. The one magnet may tend towards the other, with a power equal to that by which the difference of the attraction and repulsion of the nearer pole exceeds the difference of the opposite repulsion and attraction of the remoter pole. When the one magnet is very short in comparison with the other, its directive and attractive powers, being the sum and difference of nearly equal forces, will, therefore, become quite disproportioned. It is hence that a magnetic bar, floating on the surface of mercury, shows no tendency to advance towards the north, though it will yet turn vigorously in that direction. The directive energy of the needle thus furnishes the most correct measure of the the power of the terrestrial magnet, or of the joint forces exerted by its opposite poles. But the traversing of the needle corresponds exactly with the oscillation of a pendulum, and consequently the actuating power is always expressed by the square of the number of vibrat.which are performed in a given time. If these vib.átio, however, take place in the horizontal and not the magnetise, it is evident that a part only of the original force comes into play, and that the result is diminished by this obliquity in the proportion of the cosine of the inclination of the needle. Instructed by such views, Humboldt and Gay-Lussac proposed to explore the laws of terrestrial magnetism, during an excursion of nearly a twelvemonth, from the 15th of March 1805 to the 1st ef May 1806, through a great part of the Continent. They were favoured by the minister of the marine with a dipping needle of Borda's construction, and which had been executed by Lenoir for the voyage of Entrecasteaux. To meas are the vibrations, they had a magnetic bar suspended by a thread of raw silk, in a box with glass sides. These instruments would appear to have been susceptible of considerable delicacy. The traversing of the bar seemed not affected by any change of temperature, nor sensibly by the difference of elevation. It gave the same result at Milan, after an interval of six months; and the vibrations were as frequent on the summits of the Alps as in the plains of Italy. Sixty of these vibrations were performed at Berlin in the space of 3165 seconds, at Paris in 314, at Milan in 295, at Rome in 2814, and at Naples in 279,-all measured by a chronometer of Berthoud. The corresponding inclinations of the dipping needle observed at those places, were 69° 53', 69° 12', 65° 40′, 61° 57', and 61°35'. These, with other intermediate observations, are registered in a table, which likewise exhibits the calculated results. If the action at the magnetic equator be denoted by 10000, the intensities in the direction of the dipping needle at Berlin, Paris, Milan, Rome, and Naples, or between the latitudes of 52° 31' and 40° 50', will be represented by the successive numbers 13703, 13482, 13364, 12642, and 12745. But this computation is partly hypothetical, since it assumes the position of the magnetic equator, as deduced by M. Biot from the previous observations of La Peyrouse and Humboldt in America. Without adopting, therefore, any premature conclusion, the horizontal action of magnetism at Berlin, Paris, Milan, Rome and Naples, will be in the proportion of 1, 1.016, 1.147, 1.261 and 1.287; and the entire direct action as 1, .9840, .9575, .9226, and .9300. It is hence evident, that in proceeding towards the south of Europe, the force of magnetism gradually diminishes. Naples would seem to form VOL. XV. No. 29. K the the only exception; but this discrepancy was owing probably to local circumstances--to the attraction of the ferruginous lava and other volcanic productions of Mount Vesuvius. These results perfectly agree with the previous theory. The diminution of force, occasioned by receding from the nearer pole of the terrestrial magnet, is greater than the corresponding augmentation derived from approaching to the farther pole. The preceding data, combined with the declinations of the needle, might suffice, through the known laws of magnetism, for determining the position of those poles; and if similar observations were repeated at distant periods, the nature and circulation of terrestrial magnetism would at last be ascertained. To engage, however, with confidence in such an arduous investigation, would require nicer and more extensive observations. England may yet have the honour of completing the discovery. Suppose a delicate magnetic bar were substituted for the pendulum-spring, and thus made the prime-mover of a watch. The instrument being duly placed, its vibrations would evidently be maintained with regularity, for any length of time. Compared with a chronometer, at an interval perhaps of twenty-four hours, it would mark the number of vibrations, and therefore give the actuating power with the utmost precision. would also serve the purpose of a most correct dipping needle ; for the vibrations in the horizontal and vertical planes will form two sides of a rectangle, of which the diagonal indicates the magnetic tendency. But it The idea now suggested has been often proposed to a variety of ingenious persons, but never yet carried into execution. Ít is undoubtedly quite practicable, but would require some skill and perseverance to bring it to perfection. The trouble and expense attending the trials, with the prospect of only remote and contingent advantage, would discourage an individual from the attempt. It might especially claim the patronage of our Board of Longitude, if a projector could submit to the intrigue and solicitation required to move a body composed of such materials. Were this scheme once realized, we might expect to see, at no distant period, the phenomena of magnetism reduced almost to the same degree of certainty as the motions of the planets. 2. Memoir on the nature of the gas contained in the air-bladder of fishes. By M. Biot. It is well known that fishes are enabled to sink or rise in their native element by means of an air-bladder, which they can compress or suffer to distend at pleasure. The difficulty is to conceive how the air contained in that bag is procured. It must obviously obviously be derived from the liquid in which they swim; but whether it is extruded by mechanical action, or secreted by some process of the animal economy, naturalists have not yet decided. Air, in a certain proportion, is feebly combined with all fresh water, from which it is easily expelled by the application of heat, the removal of atmospheric pressure, the gradual attack of congelation, or the addition of soluble matters. We might hence presume, that scarcely any portion of air is lodged in salt water. Though the fishes in rivers or lakes were to extract air by the action of their gills, the inhabitants of the ocean could not be supposed to obtain their supply through the same means. The best mode, however, of settling the question, is to discover the quality of the gas contained in the bladder, and to compare it with atmospheric air. This was the object proposed by M. Biot, whose active mind embraces every philosophical pursuit. His experiments were performed at intervals, on the shores of the savage isles of Yviza and Fromentera, while engaged in extending the measurement of the meridian by a chain of triangles, from Barcelona across the Mediterranean. And it is consoling to reflect, that the academician was allowed to carry forward his interesting operations unmolested, and even under the protection of the British government, during a war which has unfortunately been prosecuted with a rancour disgraceful to civilized nations. M. Biot expressed the air-bladders, and examined the gas by means of Volta's eudiometer. The hydrogen employed was of the purest kind, and collected over boiled water. A measure of it being transferred to the graduated tube containing the peculiar gas, the mixture was inflamed by an electric spark, and the quantity of absorption noted. In this way, the proportion of oxygen was accurately ascertained. The gas from different fishes, however, appeared extremely various in its composition: it contained no hydrogen, and scarcely any carbonic acid; but, in some specimens, it consisted almost entirely of azote; and in others a small portion of azote was combined with nearly six-seventh parts of oxygen. A curious fact was detected in the course of these experiments; namely, that the fishes which reside in deep water hold a larger proportion of oxygen. The difference in this respect was so very striking, that even the sailors who assisted M. Biot in his operations, were ready to anticipate the degree of detonation produced by the electric spark, according to the depth at which the fishes had been caught. It would hence appear, that the agency of the air-bladder has only a limited range, and that the different inhabitants of the ocean affect particular depths. In fact, a fish which lives 80 fathoms under water, must sustain a pressure K 2 |