1844.1 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



339 



to move in one direction more than anotlier, precipitate itself towards 

 the earth as soon as it ceases to be upheld. Nature engenders the 

 weight of bodies by ways so concealed, so much beyond the reach of 

 our senses, and the ordinary resources of human intellect, that the 

 philosophers who, in antiquity, thought they could explain everything 

 mechanically, according to the simple evolutions of atoms, excepted 

 weight. Descartes tried what Leucippus, Democritus, Epicurus, and 

 their schools bad thought impossible. He made the fall of terrestrial 

 bodies depend on the action of a wliirlwind of very subtile matter cir- 

 culating around our globe. The real improvement which the illus- 

 trious Huygens added to the ingenious conception of our fellow-coun- 

 tryman were far, however, from giving clearness and precision to it, 

 those characti'ristic attributes of truth. Those appreciate very ill the 

 direction, tlie bearing of one of the greatest questions in which the 

 moderns have engaged, who see Newton come forth victorious from a 

 contest in which his two immortal predecessors had succumbed. 

 Newton no more discovered the cause of gravitation than Galileo had 

 done. Two bodies near each other approach. Newton did not seek 

 the nature of the power which produced this eftect. The power 

 exists, he calls it by the name of attraction, but with the warning that 

 the term from his pen implies no fixed idea touching the mode of 

 physical action, according to which gravitation arises and is brought 

 into action. Attractive force once admitted as a fact, Newton follows 

 it up and studies it in terrestrial phenomena, in the revolution of the 

 moon, planets, satellites, comets, and, as we have already said, he pro- 

 duces from this incomparable labour the mathematical, simple, and 

 universal characters of the forces which preside over the movements 

 of all the stars which compose our planetary system. The loud ap- 

 plauses of the learned world did not prevent the immortal author of 

 the Treatise on Natural Philosophy from hearing isolated voices pro- 

 nounce, as the Decision of universal attraction, the words " occult 

 qualities." This word made Newton and his most devoted and en- 

 thusiastic disciples give up the reserve which they thought it their 

 duty to observe. Then were banished to the class of the ignorant 

 those who considered attraction as an essential property of matter, as 

 the mysterious index of a sort of charm ; who supposed that two 

 bodies could act upon each other without the intermediation of a third 

 body : then this power became in every place either the resultant of 

 the ertbrt made by a certain fluid (ether), to escape into the free 

 regions of space, where its density is at its maximum, towards the 

 planetary bodies around which it exists in the greatest state of rare- 

 faction, or either the consequence of the impulse of some fluid medium. 

 Newton never explained himself categorically on the manner in 

 which an impulse, the physical cause of the attractive power of mat- 

 ter, could arise, at least in our solar system. But we have now very 

 strong reasons for believing that in writing the word impulse the great 

 geometer was thinking of the systematic ideas of Varignon and Natio 

 de Duillier, later restored and perfected by Lesage ; these ideas, in 

 fact, had been communicated to him before publication. According 

 to Lesage's ideas, there are in the regions of space corpuscles moving 

 themselves in all possible directions, and with excessive rapidity. 

 The author gave to these corpuscles the name of ultra-mondane cor- 

 puscule. Their aggregate composed the gravific fluid, if, however, 

 the designation of fluid could be applied to a collection of particles 

 having no connection together. An unique body, placed in the middle 

 of such an ocean of movable corpuscles, would remain in repose, 

 since it would be equally pushed in every direction. On the other 

 two bodies would move towards each other, for their regardant sur- 

 faces would no longer be struck, in the direction of the line which 

 would join them, by the ultra-mondane corpuscles ; for there would 

 then exist currents of which the ett'ect would no longer be destroyed 

 by counter currents. It is easily seen that two bodies placed in the 

 gravific fluid would tend to approach, with an intensity which would 

 vary in the inverse ratio of the square of the distances. If attraction 

 is the result of the impulsion of a fluid, its action should employ a 

 finite time in passing through the immense spaces which separate the 

 heavenly bodies. The sun would then be suddenly annihilated, so 

 that alter the catastrophe, mathematically speaking, the earth would 

 still feel its attraction for some time. The contrary would happen on 

 the sudden birth of a planet ; a certain time would transpire before 

 the attractive action of the new star would be felt on our globe. 

 Several geometers of the last century believed that attraction was not 

 instantaneously transmitted from one body to another; they even 

 gifted it with a very slight velocity of propagation. Daniel Bernouilli 

 for instance, wishing to explain how the highest tide arrives on our 

 coasts a day and a half after the syzygies, that is to say, a day and a 

 half after the epochs when the sun and moon have been most favour- 

 ably situated for the production of this magnificent phenomenon, 

 admits that the lunar action employed all this time (a day and a half) 

 in transmitting itself from the moon to the sea. Such a low velocity 



could not be made to agree with the mechanical explanations of the 

 weight of which we have spoken. The explanation, indeed, impe- 

 riously supposes that the proper velocity of the heavenly bodies is 

 comparatively insensible to that of the gravific fluid. 



Before having found that the actual diminution of eccentricity in 

 the earthly orbit is the real cause of the acceleration observed in the 

 movement of the moon, Laplace, on his side, had sought whether this 

 mysterious acceleration did not depend on the successive propagation 

 of attraction. Calculation for a moment made the supposition plausi- 

 ble. He showed that the gradual propagation of attraction would 

 inevitably introduce into the movement of our satellite a per- 

 turbation proportionate to the square of the time lapsed, beginning 

 with any epoch ; that to represent numerically the results of astrono- 

 mical observations, it would be by no means necessary to attribute to 

 attraction low velocities ; that a propagation eight million times more 

 rapid than that of light would satisfy all these phenomena. Although 

 the true cause of the acceleration of the moon be now well known, the 

 ingenious calculation of which I have just spoken does not the less 

 preserve its place in science. In a mathematical point of view, the 

 perturbation dependant on the successive propagation of attraction 

 which this calculation points out, has a certain existence. The con- 

 nection between the velocity and the perturbation is such that one of 

 the two quantities leads to the numerical knowledge of the other. 

 But by giving to the perturbation the maximum value which observa- 

 tions allow when they are corrected by the known acceleration arising 

 from the change of eccentricity in the earthly orbit we find for the 

 velocity of the attractive force — fifty million times the speed of light. 

 By recollecting that this number is a minimum limit, and that the 

 speed of the luminous rays equals 200,000 miles per second, those 

 philosophers who pretend to explain attraction by the impulse of a 

 fluid, win see what prodigious velocities they have to satisfy. The 

 reader will here again remark with what sagacity Laplace knew how 

 to take advantage of the phenomena best adapted to throw light on 

 the ardous questions of celestial physics ; and with what good fortune 

 he explored them, bringing forth numerical conclusions before which 

 the mind becomes confused. 



The author of the Mi'canique Citesle admitted with Newton that 

 light is composed of material molecules of excessive tenuity, and 

 gifted in free space with a velocity of 200,000 miles per second. 

 However we must warn those who would take advantage of this im- 

 posing authority that the principal argument of Laplace in favour of 

 the system of emission was the possibility of subjecting everything 

 in it to simple and rigorous calculation, while the undulatory theory 

 presented to analysis, and still oilers immense difficulties. It was ma- 

 terial for a geometer who had so elegantly connected with attractive 

 and repulsive forces, the laws of simple refraction to which light 

 obeys in the atmosphere, and of double refraction which it obeys in 

 certain crystals, should not abandon this path before having raathema.. 

 tically ascertained the impossibility of arriving in the same manner at 

 plausible explanations of the phenomena of diffraction and polarisation. 

 Besides the care which Laplace always took to push his researches as 

 much as possible to numerical deductions will permit philosophers, 

 who undertake a complete comparison of the two rival theories of 

 light, to seek in the Mrcauique Cclesle, the data of many comparisons 

 very striking and full of interest. Is light an emanation from the sun? 

 does that star dart at every moment and in all directions, a part of its 

 own substance? does it diminish gradually in mass or volume? The 

 solar attraction of our globe would then become less and less con- 

 siderable; the radius of the terrestrial orbit, on the other hand, could 

 not fail to increase, and the length of the year would receive a cor- 

 responding augmentation. That is with every one the result of a 

 first glance. By applying analytical calculation to the question, by 

 descending thus to numerical applications by the help of the more 

 precise results of observation as to the duration of the year in different 

 ages. Laplace proved that in 2000 years a constant emission of light 

 has not diminished the mass of the sun one two thousandth part of its 

 primitive value. 



Our illustrious fellow-countryman never proposed to himself any- 

 thing vague or indeterminate. His constant object was the explana- 

 lion of some grand natural phenomena, according to the inflexible rules 

 of mathematical analysis. No philosopher, no geometer more care- 

 fully kept himself in check against the spirit of systematizing. No 

 one feared more the scientific errors which imagination brings forth, 

 when it is not circumscribed with the bounds of facts, calculation and 

 analogy. Once, once only, Laplace cast himself like Kepler, like 

 Descartes, like Leibnitz, like Buffon, in the reign of conjecture. His 

 conception was then nothing less than a cosmogony. 



All planets revolve around the sun from west to east, and in planes 

 which form with each other very slight angles. The satellites move 

 around their respective planets like the planets around the sun, that 



