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Upon the whole, this Introduction to Botany seems to have been a hurried and a careless production. To us it appears not unlikely, that, in composing it, the Doctor has occasionally taken large portions of the manuscript of his lectures at the Royal Institution, and, dividing them into chapters, sept them, without more ado, to the press, as constituent parts of his book. While, therefore, it may be found a very useful assistant, it is not certainly that masterly botanical grammar which might have been expected from so eminent an author; nor calculated to suipersede the elementary treatises of Willdenow, Rose, Hull, and others.

One characteristic it certainly possesses in an eminent degreedelicacy. Those who are acquainted with the writings of Linnæus, know well how much they abound with coarse expressions and indelicate allusions. These are most scrupulously avoided by Dr Smith; and, we think, without any material detriment to the perspicuity of his descriptions. Botany is daily becoming a more fashionable female study; and this is an elementary book which

may. be put with confidence into the hands of women, without any risk of wounding the most delicate mind. We are har. py, for the sake of those fair students, to observe that Dr Smith promises a translation of his Flora Britannica; for this, we doubt not, will, when accomplished by Dr Smith himself, form the best popular herbal ever published. In the mean time, he very candidly recommends Dr Withering's Arrangement of British Plants; to which we would take the liberty to add Mr Galpine's Compend of British Botany (which is indeed nearly a translation of Dr Smith's Compendium Flora Britannica), as a most useful and commodious pocket companion in botanical excursions.

Art. IX. Memoires de Physique et Chimie, de la Societé d'Ar

cueil. Tom. I. 8vo. Paris, 1807,

THI
This volume is the production of a little association, better calcu-

lated, we conceive, than the older establishments, for advancing the progress of physical science. The celebrated Berthollet, whole Jabours have so materially contributed to extend the practice and improve the theory of chemistry, anxious, amidst the possession of ease and competence, to promote, in his declining years, the objects of his earliest ambition, has gathered around him a few ingenious and adlive individuals, who assemble once a fortnight at his gountry residence near Paris, and spend the day in philosophi..

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cal occupations. From the name of that place, the society derives
its appellation of Arcuril. Besides La Place, who appears rather
as a patron and counsellor, the members consist of the younger
Berthollet, Biot, Gay-Lussac, Humboldt, Thenard, Decandolle,
and Collet-Descostils. At their meetings, the latest fcientific jour-
nals are consulted, philosophical papers are read and difcuffed, and
new experiments are propofed, repeated, or fet on foot. The ad-
vantages of such a plan are most obvious. Mistakes may be de-
tected, errors avoided, and important lights struck out by the
collision of ideas. In the actual state of science, no experiments
are truly valuable, but those which have been performed with
the most fcrupulous precision. The art of experimenting itself
has now become fo refined, and attended with such vast expense,
as often to lye beyond the reach of individual exertion. We
are, therefore, inclined to augur favourably of a society of this
nature, which defcends to guide and aflift the details of inquiry.
If our expectations have not been fully answered, we yet dir-
cern the germs of more important communications; and trust
that similar associations, furnithed with more ample means, will
foon be formed at home. We confine our selections to those pa-
pers 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 Messieurs Humboldt and Gay-Lussac. The laws of magnetic action were first discovered by our ingenious countryman Dr Gilbert of Colchester. That original phi. losopher, who, prior to the writings of Bacon, understood and successfully pursued the method of induction, reduced all the phenomena of magnetism to four general facts: 1. That, of two magnets, the similar 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 apposition, has a power of inducing magnetism on a piece of iron or steel, and which is - more or lels durable according to the hardnefs or softness of the material affected; and, 4. That the mass of the earth itself contains an immense magnet, poffefling those distinctive properties. From such data, it was easy to explain the more obvious effects of magnetisın. A magnet attracts a bit of iron, because the iron, for the time at least, becomes likewise a magnet, endued with polarity; and a magnetic bar, freely suspended, 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 observe ed to deviate gradually to the weit, with a declination that has, for

abour

about two centuries, been continually increasing. The rate of this digression, indeed, has of late years sensibly relaxed; which affords a presumption, that, perhaps in the course of another century, the needle will have finished its period of aberration, and may return again by the same steps. The variation of the mariner's compass differs, however, in each particular place : sometimes it is stationary, but generally it is either advancing or retreating, and that with a progress unequal, and ever subject to change. Such a

system of perpetual mutation is in the highest degree perplexing, and forms one of the greatest obstacles to the practice and improvement of navigation. But all this intricacy and seeming irregularity may result from the combination of a few very simple changes. What could, at first sight, appear more involved than the motions of the heavenly bodies? And yet, when the separate elements are developed, how simple and harmonious the whole becomes ! There is the strongest reason, therefore, to conclude, that the complicated aberrations of the needle may proceed from certain regular changes in the position of the poles of the terrestrial magnet. · The great desideratum is now to ascertain the nature of those changes. For that purpose, it is necessary to determine accurately, at distant points on the earth's surface, the direction and intensity of the magnetic action. Hitherto, the direction only has been observed, by help of the compass and dipping needle; and even these observations, from the imperfection of the latter instrument, are in many cases doubtful or defective. To discover the relative intensities of magnetic action at different places, would lead more immediately to the solution 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 excrted by its o; positi poles. But the traversing of the needle correspondis exactly with the oscillation vé a penduium, and consequently the actuatirgo is always expressed by the square of the number of vibrat. *, which are performed in a given time. If these vib_stico many nowever, take place in the horizontal and not the mag op d'e, it is evident that a part only of the original force curies 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 twelvenontn, f.om the 15th of March 1805 to the 1st cf May 1906, through a great part of thic Continent. They were favoured by the minister of the marine with a dippisneedle of Borda’s construction, and which had been executed by Lencir for the voyage of En secasteaex, To meas . re the vibratior.s, they had a magnetic bar suspended by a thread of raw silk, ha 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 3164 seconds, at Paris in 314, at Milan in 2957, at Rome in 2014, and at Naples in 279,--all measured by a chronometer of Bera thoud. 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° 50,4', will be represented by the successive numbers 13703, 13482, 13364, 12012, 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 POL. XY, NO. 29.

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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 correspondiog augmentation derived from approaching to the farther pole. The preceding data, combined with the declinations of the needle, might suslice, 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 inagnetism 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. But it 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.

The idea now suggested has been often proposed to a variety of ingenious persons, but never yet carried into execution. It 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

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