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Let us cite two or three of the laws of Laplace:
If we add to the mean longitude of the first satellite twice that of the third, and subtract from the suin three times the mean longitude of the second, the result will be exactly equal to 1800. Would it not be very extraordinary if the three satellites had been placed originally at the distances from Jupiter, and in the positions, with respect to each other, adapted for constantly aud rigorously maintaining the foregoing relation? Laplace has replied to tbis question by showing that it is not vecessary that this relation should have been rigorously true at the ori. giil. The mutual action of the satellites would necessarily have reduced it to its present mathematical condition, if once the distances and the positions satisfied the law approximately.
This first law is equally true when we employ the synodical elements. It hence plainly results that the three first satellites of Jupiter can never be all eclipsed at the same time. Bearing this in mind, we sball bave no difficulty in apprehending the import of a celebrated observation of recent times, during which certain astronomers perceived the planet for a short time without any of his four satellites. This would not by any means authorize us in supposing the satellites to be eclipsed. A satellite disappears when it is projected upon the central part of the lumi. nous disk of Jupiter, and also when it passes behind the opaque body of the planet.
The foilowing is another very simple law to which the mean motions of the same satellites of Jupiter are subject:
If we add to tbe mean inotion of the first satellite twice the mean motion of the third, the sum is exactly equal to three times the mean motion of the second. *
This numerical co-incidence, which is perfectly accurate, would be one of the most mysterious phenomena in the system of the universe if
* This law is necessarily included in the law already enunciated by the author relative to the mean longitudes. The following is the most usual mode of expressing these curious rela. tions : Ist, the mean motion of the first satellite, plus twice the mean motion of the third, miuus three times the mean motion of the second, is rigorously equal to zero; 2d, the mean longitude of the first satellite, plus twice the mean longitude of the third, minus three times the mean longitude of the second, is equal to 1800. It is plain that if we only consider the niean longitude here to refer to a giren epoch, the combination of the two laws will assure the existence of an analogous relation between the mean longitudes for any instant of time whatever, whether past or future. Laplace has shown, as the author has stated in the text, that if these relations had only been approximately true at the origin, the mutual attraction of the three satellites would have ultimately rendered them rigorously so: under such circumstances the mean longitude of the first satellite, plus twice the mean longitude of the third, minus three times the mean longitude of the second, would cuntinually oscillate about 1800 as a mean value. The three satellites would participate in this libratory movement, the extent of oscillation depending in each case on the mass of the satellite and its distance from the primary, but the period of libration is the same for all the satellites, amounting to 2,270 days 18 hours, or rather more than six years. Observations of the eclipses of the satellites have not afforded any indications of the actual existence of such a libratory motion, so that the relations between the mean motions and mean longitudes may be presumed to be always rigorously true.- TRANSLATOR.
Laplace had not proved that the law need only have been approximate at the origin, and that the mutual action of the satellites has sufficed to render it rigorons.
The illustrious geometer who always pursued his researches to their most remote ramifications arrived at the following result: The action of Jupiter regulates the movements of rotation of the satellites, so that, without taking into account the secular perturbations, the time of rotation of the first satellite, plus twice the time of rotation of the third, forms a sum which is constantly equal to three times the time of rotation of the second. Influenced by a deference, a modesty, a timidity, without any plausible motive, our artists in the last century surrendered to the English the exclusive privilege of constructing instruments of astronomy. Thus, let us frankly acknowledge the fact, at the time when Herschel was prosecuting his beautiful observations on the other side of the Channel, there existed in France no instruments adapted for developing them ; we bad not even the means of verifying them. Fortunately for the scientific honor of our country, mathematical analysis is also a powerful instrument. Laplace gave ampie proof of this ou a memorable occasion when, from the retirement of bis chamber, he predicted, he minutely announced, what the excellent astronomer of Windsor would see with the largest telescopes which were ever constructed by the hand of man.
When Galileo, in the beginning of the year 1610, directed toward Saturn a telescope of very low power, which he had just executed with his own hands, he perceived that the planet was not an ordinary globe, without, however, being able to ascertain its real form. The expression "tri-corporate," by which the illustrious Florentive designated the appearance of the planet, implied even a totally erroneous idea of its structure. Our countryman Roberval entertained much sounder views on the subject, but froin not having instituted a detailed comparison between bis hypothesis and the results of observation, he abandoned to Huyghens the honor of being regarded as the author of the true theory of tile phenomena presented by the wonderful planet. Every person knows in the present day that Saturn consists of a globe about 900 times greater than the earth, and a ring. This ring does not touch the ball of the planet, being everywhere removed from it at a distance of 20,000 (English) miles. Observation indicates the breadth of the ring to be 54,000 miles. The thickness certainly does not exceed 250 miles. With the exception of a black streak, which divides the ring througbout its whole contour into two parts of unequal breadth and of different brightness, this strange, colossal bridge without piles had never offered to the most experienced or skillful observer either spot or protuberance adapted for deciding whetber it was immovable or endued with a movement of rotation.
Laplace considered it to be very improbable, if the ring was immovable, that its constituent parts should be capable of resisting by their
mere cohesion the continual attraction of the planet. A movement of rotation occurred to his mind as constituting the principle of stability, and he hence deduced the necessary velocity. The velocity thus found was exactly equal to that which Herschel subsequently deduced from a course of extremely delicate observations.
The two parts of the ring being placed at different distances from the planet, could not fail to experience, from the action of the sun, different movements of rotation. It would bence seem that the planes of both rings ought to be generally inclined toward each other, whereas they appear from observation always to coincide. It was necessary, then, that some pbysical cause should exist which would be capable of neutralizing the action of the sun. In a memoir published in February, 1789, Laplace found that this cause must reside in the ellipticity of Saturn, produced by a rapid movement of rotation of the planet, a movemeut the existence of which Herschel announced in November, 1789.
The reader cannot fail to remark how, on certain occasions, the eyes of the mind can supply the want of the most powerful telescopes, and lead to astronomical discoveries of the highest importance. Let us descend from the heavens upon the earth. The discoveries of Laplace will appear not less important, not less worthy of his genius.
The phenomena of the tides, which an ancient philosopher designated in despair the tomb of human curiosity, were conuected by Laplace with an analytical theory in which the physical conditions of the question figure for the first time. Accordingly calculators, to the immense advautage of the navigation of our maritime coasts, venture in the present day to predict several years in advance the details of the time and height of the full tides without more auxiety respecting the result tban if the question related to the phases of an eclipse.
There exists between the different phenomena of the ebb and flow of the tides and the attractive forces which the sun and moon exercise upon the fluid sheet which covers three-fourths of the globe, au intimate and necessary connection, from wbich Laplace, by the aid of a series of twenty years of observations, executed at Brest, deduced the value of tbe mass of our satellite. Science knows in the present day that seventytive moons would be necessary to form a weight equivalent to that of the terrestrial globe, and it is indebted for this result to an attentive and minute study of the oscillations of the ocean. We know only one ineans of enhancing the admiration which every thoughtful mind will entertain for theories capable of leading to such conclusions. Au bistorical statement will supply it. In the year 1631, the illustrious Galileo, as appears from his Dialogues, was so far from perceiving the mathematical relations from which Laplace deduced results so beautiful, so unequivocal, and so useful, that he taxed with frivolousness the vague idea which Kepler entertained of attributing to the moon's attraction a certain share in the production of the diurnal and periodical movements of the waters of the ocean.
Lapiace did not confine himself to exteuding so considerably, and improving so essentially, the mathematical theory of the tides; he considered the phenomenon from an entirely new poiut of view; it was he who first treated of the stability of the ocean. Systems of bodies, whether solid or fluid, are subject to two kinds of equilibrium, which we must carefully distinguish from each other. In the case of stable equilibrium, the system, wheu slightly disturbed, tends always to return to its origiual condition. Ontbe other hand, when the system is in unstable equilibrium, a very insiguificant derangement might occasion an enormous dislocation in the relative positions of its constituent parts.
If the equlibrium of waves is of the latter kind, the waves engendered by the action of winds, by earthquakes, and by sudden movements from the bottom of the ocean, have perhaps risen in past times, and may rise in the future, to the height of the liighest mountains. The geologist will have the satisfaction of deducing from these prodigious oscillations a rational explanation of a great multitude of phenomena, but the public will thereby be exposed to new and terrible catastropbes.
Mankind may rest assured ; Laplace has proved, that the equilibrium of the ocean is stable, but upon the express condition (wbich, however, has been amply verified by established facts) tbat the mean density of the fluid mass is less than the mean density of the earth. Everything else remaining the same, let us substitute an ocean of mercury for the actual ocean, and the stability will disappear, and the fluid will frequently surpass its boundaries, to ravage continents even to the height of the snowy regions wbich lose themselves in the clouds.
Does not the reader remark how each of the analytical investigations of Laplace serves to disclose the harmony and duration of the universe and of our globe ?
It was impossible that the great geometer, who had succeeded so well in the study of the tides of the ocean, should not have occupied his attention with the tides of the atmospbere ; that he should not have submitted to the delicate and definitive tests of a rigorous calculus the generally-diffused opinions respecting the influence of the moon upon the height of the barometer and other meteorological phenomena.
Laplace, in effect, bas devoted a chapter of his splendid work to an examination of the oscillations which the attractive force of the moon is capable of producing in our atmosphere. It results from these researches that, at Paris, the lunar tide produces no sensible effect upon the barometer. The height of the tide, obtained by the discussion of a long series of observations, has not exceeded two-hundredths of a millimeter, a quantity wbich, in the present state of meteorological science, is less than the probable error of observation.
The calculation to which I have just alluded may be cited in support of considerations to which I bad recourse when I wished to establish, that if the moon alters more or less the height of the barometer, according to its different phases, the effect is not attributable to attraction.
No person was more sagacious than Laplace in discovering intimate relations between phenomena apparently very dissimilar; no person showed himself more skillful in deducing important conclusions from those unexpected affinities..
Toward the close of his days, for example, he overthrew with a stroke of the pen, by the aid of certain observations of the moon, the cosmogonic theories of Buffon and Bailly, which were so long in favor. According to these theories, the earth was inevitably advancing to a state of congelation which was close at hand. Laplace, who never contented himself with a vague statement, sought to determine in numbers the rapid cooling of our globe which Buffon bad so eloquently but so gra. tuitously announced. Nothing could be more simple, better connected, or more demonstrative than the chain of deductions of the celebrated geometer.
A body diminishes in volume when it cools. According to the most elementary principles of mechanics, a rotating body which contracts in dimensions ought inevitably to turn upon its axis with greater and greater rapidity. The length of the day has been determined in all ages by the time of the earth's rotation ; if the earth is cooling, the length of the day must be continually shortening. Now, there exists a means of ascertaining whether the length of the day has undergone any variation ; tbis consists in examining, for each century, the arc of the celestial sphere described by the moon during the interval of time which the astronomers of the existing epoch called a day; in other words, the time required by the earth to effect a complete rotation on its axis, the velocity of the moou being, in fact, independent of the time of the earth's rotation.
Let us now, after the example of Laplace, take from the standard tables the least considerable values, if you choose, of the expansions or contractions wbich solid bodies experience from changes of temperature; search then the annals of Grecian, Arabian, and modern astronomy for the purpose of finding ju them the angular velocity of the moon, and the great geometer will prove, by incontrovertible evidence, founded upon these data, that during a period of 2,000 years the mean temperature of the earth has not varied to the extent of the hundredth part of a degree of the centigrade thermometer. No eloquent declamation is capable of resisting such a process of reasoning, or withstanding the force of such numbers. The mathematics have been in all ages the implacable adversaries of scientific romances.
The fall of bodies, if it was not a phenomenon of perpetual occurrence, would justly excite in the highest degree the astonishment of mankind. What, in effect, is more extraordinary than to see an inert mass—that is to say, a mass deprived of will, a mass which ought not to bave any propensity to advance in one direction more than in another, precipitate itself toward the earth as soon as it ceased to be supported ?
Nature engenders the gravity of bodies by a process so recondite, so