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THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



[SEPTBMBSft, 



third, forms a sum constantly equal to thrice the duration of the rota- 

 tion of the second. 



By a deference, modesty, and timidity, without any plausible grounds, 

 our workmen, in the last century, had given up to the English the mo- 

 nopoly of the construction of astronomical instruments. Thus let us 

 openly acknowledge it, at the time when Herschel on the other side 

 of the channel made his beautiful observations, there existed in France 

 no means of following them and developing them ; we had not even 

 the means of verifying them. Happily for the scientific honour of our 

 country, mathematical analysis is a powerful instrument. Laplace 

 provtd it so well that on a solemn occasion he foresaw from the depths 

 of his study, and minutely announced, what the skilful astronomer of 

 Windsor was going to see by making use of the largest telescopes 

 which had ever come from the hand of man. When Galileo, in the 

 beginning of 1610, directed toward Saturn a very weak telescope re- 

 cently made by his own hands, he saw that this planet is only an or- 

 dinary globe, without however being able to give an exact account of 

 its real form. The expression tri-corpus, by which the illustrious 

 Florentine philosopher summed up his reflections, implied an idea 

 completely erroneous. Our fellow-countryman Roberval was much 

 more hapjjily inspired ; but from want of having given a detailed com- 

 parison of his hypothesis and his observations, he abandoned to Huy- 

 gens the honour of being considered the anther of the true theory of 

 the phenomena which this admirable planet presents. Everybody now 

 knows that Saturn is composed of a globe ",)00 times larger than the 

 earth, and of a ring. This ring does not touch the inner globe at any 

 point, it is everywhere removed 20,001) miles. Observations carry 

 the breadth of the ring to 30,000 miles. The thickness is certainly 

 not 250 miles. Except an obscure streak, which, going through the 

 whole extent of the ring, divides it into two parts of unequal breadth 

 and dissimilar brightness, this strange colossal bridge without piers 

 had never presented to the most experienced and most able observers 

 either spot or pertuberancc capable of deciding whether it were im- 

 movable or gifted with a rotary movement. Laplace considered that 

 it was little probable, if the ring were immovable, that its constituent 

 parts should resist, by their simple adherence, the attractive and con- 

 tinual action of the planet. A movement of rotation suggested itself 

 to his mind as the conservative principle, and he determined the re- 

 quisite speed; the speed thus calculated is equal to that which Her- 

 schel deduced later from extremely delicate observations. The two 

 parts of the ring, being placed at different distances from the planet, 

 could not fail to be affected from the action of the sun with different 

 movements of precession. The planes of the two rings should thus, 

 it seemed, be generally inclined to each other, while observation in- 

 cessantly shows them confused together. It was then requisite that a 

 cause should exist capable of neutralizing the solar action. In a paper 

 published in Feb. 1789, Laplace found that this cause must be the 

 flattening of Saturn, produced by a rapid rotary motion of that planet, 

 of which Herschel announced the existence in August 1789. It will 

 be remarked how the eye of the mind can, in certain cases, supply the 

 most powerful telescopes, and lead to astronomical discoveries of the 

 highest order. 



Let us descend from heaven to earth. The discoveries of Laplace 

 will be found neither less important nor less worthy of his genius. 

 The tides, that phenomena which an ancient in despair called " the 

 tomb of human curiosity," have been, by Laplace, connected with an 

 analytical theory, in which the physical conditions of the question 

 figure for the first time. Thus calculators, to the great benefit of our 

 maritime coasts, hazard themselves now in foretelling several years in 

 advance the circumstances of hour and height of great tides, without 

 any further disquietude as to the result, than if it concerned the phases 

 of an eclipse. There exists between the phenomena of the flow, ebb, 

 and alternative actions which the sun and moon exercise on the liquid 

 stratum vfhich covers three quarters of the globe, an intimate neces- 

 sary connection, in which Laplace, making use of twenty years obser- 

 vations at Brest, determined the value of the mass of our satellite. 

 Science now knows that 75 moons would be requisite to form a weight 

 equivalent to that of our earthly globe, and this is due to the atten- 

 tive and minute study of the oscillations of the ocean. We only know 

 of one means of adding to the profound admiration which all attentive 

 minds will doubtless feel for theories susceptible of such consequences. 

 An historical quotation will supply us with it : we will recall that in 

 1G31, in his celebrated Dialogues, the illustrious Galileo was far from 

 seeing the mathematical connections whence Laplace deduced such 

 beautiful, such evident, and such useful results that he charged as 

 intptia the loose conception of Kepler of attributing to lunar attrac- 

 tion a certain part in the daily and periodical movements of the waves. 

 Laplace did not confine himself to extending them so widely, to per- 

 fecting in a manner so essential the mathematical theory of the waves ; 

 he considered further the phenomenon under quite a new light; it was 



lie who first treated of the stability of the equilibrium of the sea. The 

 systems of solid or liquid bodies are subject to two kinds of equili- 

 brium, which must be carefully distinguished. In the former, in firm 

 or stable equilibrium, the system slightly removed from its primitive 

 ))Osition, incessantly tends to return to it. In the unstable equilibrium, 

 on the other hand, a slight shock in the beginning may, in the long 

 run, become enormous. If the equilibrium of the waves is of the 

 latter kind, waves engendered by the action of the wind, by earth- 

 quakes, and by sudden movements at the bottom of the sea, might, in 

 the end, raise themselves to the height of the highest mountains. The 

 geologist would have the satisfaction of seeking in these prodigious 

 oscillations for rational explanations of a great number of phenomena, 

 but the world would be exposed to new and terrible cataclysons. 

 People may be comforted ; Laplace has proved that the equilibrium 

 of the ocean is stable, but on the express condition, elsewhere estab- 

 lished by certain facts, that the mean density of the liquid mass be 

 inferior to the mean density of the earth. For the actual sea always 

 remaining in the same state, let us substitute an ocean of mercury, 

 and stability will have disappeared, and the liquid will frequently 

 leave its bounds to devastate continents even in the snowy regions 

 lost ill the clouds. Do we not remark how every :inalytical research 

 of Laplace has shown, in the universe and in our globe, conditions of 

 order and durability. 



It was impossible that the great geometer, who had so well suc- 

 ceeded in the study of the ocean tides, should not study the tides of 

 the atmosphere; that he should not subject to the delicate and defini- 

 tive proofs of rigorous calculation, the opinions, generally spread, 

 touching the influence of the moon on the height of the barometer, 

 and on other meteorological phenomena. Laplace, in truth, has de- 

 voted a chapter of his beautiful work to the examination of the fluc- 

 tuations which the attractive force of the moon can effect on our at- 

 mosphere. It results from these researches that at Paris the lunar 

 flux measured on the barometer is nowise sensible. The value of this 

 flux, obtained by the discussion of a long series of observations, has 

 not exceeded two hundredths of a millimeter, (-nrmr of an inch,) a 

 quantity inferior to those for which it is possible to answer in the 

 actual state of meteorological science. The calculation to which I 

 have just referred may be adduced in support of the considerations to 

 which I had recourse in another article of the Annuaire, when I en- 

 dpavoured to establish that if the moon modifies, more or less, accord- 

 ing to its dirt'erent phases, the height of the barometer, it is not by 

 attraction, 



No one w'as ever more ingenious than Laplace in laying hold of the 

 relations and intimate connections between phenomena apparently 

 ditt'erent; no on" showed more ability in drawing important conclu- 

 sions from these unexpected comparisons. Toward the end of his 

 days, for instance, he upset by a stroke of his pen, with the help of a 

 few observations of the moon, the cosmogonic theories of Bulfon and 

 Bailly, so long in vogue. According to these theories the earth 

 moved towards an inevitable and approiiching congelation. Laplace, 

 who never contented himself with a vague expression, endeavoured 

 to determine by numbers the great speed of cooling in our globe, 

 which Button had so eloquently, but so gratuitously announced. No- 

 thing could be more simple, belter connected, or more demonstrative, 

 than the chain of deductions of the celebrated geometer. A body 

 lessens in its dimensions when it cools. According to the most ele- 

 mentary principles of mechanies, a rotary body which contracts must 

 inevitably turn faster and fast' r. Tiie day at all periods has had for 

 its duration the time of the earth's rotation; if the earth cooled down 

 the day must incessantly shorten. But there is a means of discovering 

 whether the duration of the day has varied: it is to examine in cacli 

 century what has been the arc of the celestial sphere which the moon 

 has traversed during the time that the astronomers of the period called 

 a day, during the time that the earth employs to make a revolution 

 on itself; the speed of the moon being in truth independent of the 

 duration of the rotation of our globe. Now take with Laplace, in 

 known tables, the slightest values, if you like, of the dilatations or con- 

 tractions to which solid bodies are subject from changes in tempera- 

 ture ; then search in the annals of Greek, Arab, and modern astronomy 

 to find the anguhir velocity of the moon, and the great geometer will 

 from these data bring the invincible proof that in 2UU0years the mean 

 temperature of the globe has not varied the hundredth part of a cen- 

 tigrade degree. There is no efi'ect of eloquence which can resist the 

 authority of a similar argument, the power of such figures. Mathe- 

 matics have in all times been implacable adversaries of scientific 

 romances. 



The fall of bodies, if it were not a phenomena of every moment, 

 would excite justly, and in the highest degree, the astonishment of 

 men. What is more extraordinary, indeed, than to see a mass inert, 

 that IS to say deprived of wiil| a mass which can have no propensity 



