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versities, and in the Faculties of Divinity and of Canon Law; and
The above-mentioned letter to Innocent was written only
We scarcely think that the French clergy of the present day will thank this writer, for his unseasovable assault on those lie berties which their ancestors so ably defended against the encroachments of ambitious pontiffs, who wished to enslave the Christian world, and aspired at universal dominion. The author assures us that he is neither a Protestant nor a Jansenist; and to this declaration we give full credit.
Art. V. Exposition du Système du Monde, &c. i.e. An Illustratio2
of the System of the World. By Pierre SIMON LA PLACE,
4to. 2 Vols. Paris.
1793, and a liue account of it was promised for the next
The design of the author, who, to acute penetration and the most profound knowlege of analysis, joins no inconsiderable share of eloquence, may be understood from his intro. duction :
· There is a wide interval (he observes) between the first view of the heavens, and the comprehensive view of modern times which embraces both the past and Cuture states of the system of the world. To arrive at a prospect so comprehensive, it has been necessary to observe the heavenly bodies during a great number of ages; to distinguish, in their appearances, the real motions of the earth; to zo. cend to the laws of planetary motions, and from these laws to the principle of universal gravitation; and finally 11 descend from this principle to the full explanation of the heavenly phænomena, even in their minutest details. This has human genius effected in astronomy. The explanation of the discoveries, and of the most simple manner by Min 2
which they were produced, tlie one from the other, will have the twofold advantage of presenting a grand totality of important truths, and the true method to be pursued in the investigation of the laws of pature: this is the object which I propose to myself in the present undertaking.'
The work is divided into chapters. The ist discusses the diurnal motions of the heavens; in which the phenomena that ordinarily present themselves are noted and explained. This chapter merits the highest praise for the ease of its style, for its perspicuity, and for its arrangement. Chapter II. treats on the Sun, and his proper motion, and is equally meritorious. Chapter III. on Time, and its measure. Here the author treats of the new division ; and, speaking of the necessity of fixing a proper ara, he says
• It is desirable that all people should adopt one and the same æra, independant of moral revolutions, and founded solely on astronomical plænomena. The origin of this ara might be fixed in the year ia which the
of the solar orbit coinci:led with the solar solstice, which ascends as high as the year 1250. This origia might be taken in the instant of the mean vernal cquinox, which in this
year swers to the 5th of March, 5. 3676, at Paris. The universal meridian, where the origin of the terrestial longitudes might be fixed, would be that of the place which reckoned midnight at the same it. stant, and which is to the east of Paris 135o. 2960. If, after a long series of
years, the origin of the æra became uncertain, it would bu difficult io firid it by the sole movement of the apogee, on account of the slowness and the inequalities of this movement : but all unctie tainty respecting this origin, and the position of the universal meridian, will be removed when it is remembered that, at the moment of the mean equinos, the mean longitude of the moon was 143: 7716 Thus what is arbitrary in the origin of time, and in that of the ici restial longitudes, might be made to disappear. In adopting after. ward the intercalation, and the preceding division of the year, and that of the month and clay, the calendar would be formed in the most natural and most simple manner that can be devised, for inhabitants on this side of the equator.'
Chap. IV. On the Moon's mean motion ; her phases and eclipses.
Chap. V. Of the planets; particularly Mercury and Venus.
Chap. VI. Of Mars. VII. Of Jupiter and his satellites. VIII. Of Saturn, his satellites and ring. IX. Of Uranus and his satellites. X. Of Comets. XI. Of Stars, and their motions. XII. Of the figure of the Earth, and the variation of gravity at its surface. In this chapter, the new system of weights and measures is explained with great perspicuity, and justified; and we only omit to give an extract of ihe author's clear and forcible reasoning, because we recollect that the * 3
system has already been presented to the public in Mr. Nicholson's valuable Journal : see M. R. July, p. 301 & seq.
Chap. XUI. Of the flux and rellux of the Sea. XIV. Of the terrestial Atmosphere, and astronomical Refractions.
The Second Book treats on the real motions of the heavenly bodies. Chap. I. Of the movement of the Earth’s rotation. Chap. II. Of the motion of the planets about the Sun. III. Of the motion of the Earth about the Sun. The second and third of these chapters deserve notice as specimens of luminous and forcible argumentation. We shall give an extract from the latter.
Now, shall we suppose the sun to be accompanied by planets and satellites in motion round the eartli ; or shall we make the earth move, as the planets, round the sun? The appearances of the heavenly motions are the same in the two hypotheses: but the second ought to be preferred, for the following considerations.
The masses of the sun, and of several of the planeis, being considerably greater than the mass of the earth; it is much more simple to make the latter revolve about the sun, than to put in motion round At the whole solar system. What complication in the heavenly motions does the immability of the earth induce? How rapid must be the motion which we must then attribute to Jupiter, to Saturn almost ten times more distant froni the sum, to the planet Uranus stiil more distant ; when they are made to move, each year, about us, and at the same time about the sun ? This complication, and this rapi, dity of motion, disappear by the motion of the earth's translation ; a motion conformable to the general law, according to which the sinall heavenly bodies revolve round the large bodies to which they are near,
• The analogy of the earth to the planets confirms this motion. As to Jupiter, it revolves round itself, and is accompanied by a satellite. An observer, at the surface of Jupiter, would deem the solar system in motion round him ; and the greatness of this planet would render this illusion less improbable than in the case of the earth. Is it not natural to think that the motion of this system, about us, is probably only an appearance ? Let us in imagination transport ourselves to the surface of the sun, and thence contemplate the earth and the planets. All these bodies will to us appear moving from west to east; and this identity of direction indicates the motion of the earth: but what proves it evidently, is the law which exists between the times of the planets' revolutions and their distances froin the sun. They revolve round him so much the more slowly, as they are more removed and regulated by this law, that the squares of the times of their revolutions are as the cubes of their mean distances, According to this remarkable law, the duration of the carth’s revolution, when it is supposed to move round the sun, ought to be precisely that of the sidereal year. Is not this an incontestable proof that the earth revolves like all the planets, and is subjected to the
• In other respects, would it not be strange to suppose the tera restial globe scarcely sensible to sight from the sun, tixed in the
same laws ?
midst of planets moving round this star, which itself would revolve with them about the earth? Ought not the force, which, in order to retain the planets in their respective orbits round the sun, balances their centrifugal force,-ought it not to act equally on the earth, and the earth to oppose to this action the same centrifugal force ? Thus the consideration of the heavenly motions observed from the sun leaves no doubt of the earth's real motion :--but the observer, placed on it, has an additional sensible proof of this motion, in the phænomenon of the aberration of light, which is a necessary consequence of it.We now proceed to develope this phænomenon.'
The author then gives the history of the discovery of the propagation of light, and of the aberration of the fixed stars; after which, he thus continues his argument for the truth of the Copernican system :
• The consideration of the celestial motions conducts us then to displace the earth from the centre of the world, where we suppose it to be, misled by appearances, and by the propensity which man has to consider himself as the principal object in Nature.' The globe which he inhabits is a planet moving round an axis of its own, and round the sun. In viewing it under this aspect, all the phænomena are ex. plained in the most simple manner; the laws of the heavenly motions are uniform; and all analogies are observed. Like Jupiter, Saturn, and Uranus, the earth is attended with a satellite; it revolves round itself, as do Venus, Mars, Jupiter, Saturn, and probably the other planetz ; it borrows, as they do, its light from the sun, and revolves round the sun, observing the same direction and laws. In fine, the idea of the motion of the earth unites in its favour, simplicity, ana. logy, and generally all that characterises the true system of Nature. In following it in its consequences, we shall see the celestial phæno. mena referred, even in their minutest details, to one law alone, of which they become the necessary developements. Thus will the earth's motion acquire all the certainty of which physical truths are susceptible ; and which results, whether we consider the great pumber and variety of the phænomena explained, or the simplicity of the laws from which they are made to depend. No branch of natural knowlege unites in a higher degree these advantages, which the system of the world, grounded on the earth's motion, possesses. This motion enlarges the universe in our eyes; to measure the distances of the heavenly bodies, it affords is an immense base, the dia. meter of the earth's orbit. By its means, the dimensions of the planetary orbit have been exactly determined. Thus, the earth's inotion, which by :he illusions, to which it gave rise, during a long period, retarded the knowleye of the real motions of the planets, has at lengi wade them known to us, with greater precision than if we had been placed in the focus of these motions. Nevertheless, the annual parallax of the stars, or the angle under which the diameter of the earth's orl it woull be seen from their cuntre, is iosensible, and does not an out to six seconds, even in those stars which by their supsior brig:11155 mp3lea: most near to us: these stars are then at the last a hundred i buusand times inore distant than the sun. So
prodigious a distance, joined to their brightness, is a sufficient proof that they do not, like the planets and the satellites, derive their light from the sun, but that they shine with their own light, and are as so many suns scattered in the immensity of space, which may be the foci of planetary systems. In fact, it is sufficient to place ourselves on the nearest of these stars, in order to see the sun only as a luminous star, whose diameter is less than the thirtieth part of a second
• It results from the immense distance of the stars, that their mo. tions in right ascension and declination are only appearances produced by the motion of the earth’s axis of rotation :--but some stars appear to have real and proper motions; and it is probable that they are all in motion, as the sun is; which carries along with him, througlı space, the entire system of planets, comets, and satellites; in the same manner in which each planet draws his satellites along with him in his motion round the sun.'
Chap. IV. Of the appearances which belong to the motion of the Earth. V. Of the figure of the orbits of the Planets, and the laws of their motion about the Sun. VI. Of the figure of the orbits of the Comets, and the laws of their miotion about the Sun. VII. Of the laws of the motions of Sa: tellites about their Planets.
In the Third Book, we have the author's comment on the laws of motion ; and here agaiii we think it proper to give an extract : but it will be short:
From amid the infinite variety of phænomena, which follow one another on the earth in continual succession, philosophers have at length discovered the small number of general laws which matter obeys in its motions. To these laws, every thing in Nature is submissive ; from them, every thing is derived, as necessarily as the return of seasons; and the curve described by the lightest atom, which the winds seem to carry at the caprice of chance, is regulated as cer. tainly as the planetary orbits. It should seem that the importance of these laws, on which we perpetually depend, ought to have ex: cited curiosity in every period of time: but that indifference, which is too common to the human mind, kept then concealed until the commencement of the last century; an epoch in which Galileo laid the foundations of the science of motion, by his discoveries cone cerning the fall of bodies. Geometricians, in following the rouie of this great man, have at length reduced all mechanics io certain general formulas, so exact and comprehensive, that nothing is wanting to them except the perfection of analysis.'
We have next a chapter on Forces, and their composition. This is followed by another on the motion of a material point; a chapter pregnant with just observations and luminous reasoning. We come then to remarks on the equilibrium of a system of bodies; and a chapter on the Equilibrium of Fluids. Here the author, speaking of the Forces vives, and the principle of the least action, is led to offer the following remarks on the doctrine of final causes :