248 



NA TURE 



[7?//)/ 30, 1874 



reality according to its progress in its orbit round the 

 sun, S. Evidently there is no resemblance between the=e 

 two series of figures. Then the preceding theory fails in 

 some point, and as the error will not have been in the 

 part attributed to attraction, it must be found in the 

 assumption that this is the only force. In other words, 

 it is sufficient to compare the effects of attraction with 

 the real facts, to be convinced that there must be another 

 force at work in the cometary phenomena. And as the 

 former would be capable only of disseminating the matter 

 along the orbit, the new force must be capable of driving 

 this same matter in the direction of the radius vector ; it 

 must then be opposed to attraction ; it must repel and 

 not draw. What may this force be ? Ought it not to 

 make itself felt elsewhere than in the gigantic tails of 

 comets ? How can the same body, the sun, at once 

 attract and repel matters of the same origin ? And how 

 does it come to pass that since it acts so powerfully on the 

 matter of these bodies, this repelling force of the sun 

 does not change the movement of their nuclei which 

 appear to follow so faithfully the laws of solar attraction 1 

 This last question will put us on the right tr.ack. 



And first, do comets follow rigorously, like planets, the 

 liws of attraction ? That the law has been firmly csta- 



cording to these laws, when we have taken account of the 

 perturbations caused by the neighbouring planets, the 

 time of revolution ought to be constant, while, in fact, it 

 diminishes regularly during each revolution ; the effect 

 established in this instance is of considerable magnitude, 

 about half a day. 



In face of such a fact there is room for the question 

 under consideration, viz., Is attraction the only force 

 which governs the universe ? But how can we formulate 

 such a doubt, when the carefully-studied movements of 

 the planets may be perfectly accounted for, for thousands 

 of years past, exclusively by the theory of attraction ? We 

 can escape the difficulty by an artifice identical with that 

 which enabled Newton to account for the tails of comets 

 by attraction alone : I refer to that vast and rare atmo- 

 sphere which Newton placed in space around the sun, 

 and in the midst of which the cometary materials are 

 i-levated, according to him, exactly as the smoke of our 

 chimneys in our terrestrial atmosphere. Geometers, 

 then, introduced the resistance which this general 

 medium ou^ht to oppose to the progress of a comet on 

 account of its small density, while the same medium 

 would opposo only an insensible resistance to the planets 



Wished in the case of the planets, I cannot doubt, for we 

 have for these bodies a historic series of observations 

 going back to the Chaldeans and including thousands of 

 revolutions of each of them. If there had been the 

 least disagreement between the phenomena and the law 

 to which they are assigned, the disagreement, no matter 

 how small, must at length have become sensible, after 

 accumulating during so lengthened a period. But comets, 

 in general, appear only once ; we only see them and can 

 only observe them in a very restricted part of their orbit ; 

 so that should a very slight influence alter their move- 

 ments, its effect would be confounded with the inevitable 

 errors of observation, and astronomers would not be able 

 to distinguish it. There are, no doubt, some periodic 

 comets, such as those of Halley, Bidla, Encke, c&c, but 

 the first has a period of seventy-five years, so that in going 

 back to its earlier appearances, we very soon reach the 

 time when comets belonged to the domain sf astrology. 

 That of Bidla has a period of 6J years, but its first cer- 

 tain appearance dates only from the end of last century, 

 and in the course of that time a singular accident has hap- 

 pened to it : it has been divided in two. There remains 

 Encke's comet, the only one which can be subjected to 

 the verification of which we have spoken, on account of 

 the numerous revolutions which it has accomplished since 

 its discover)' in 1786. Well, it is found that this comet, 

 the only one which can be tested in the way we speak 

 of, does not follow exactly the laws of gravitation. Ae- 



on account of their relatively small volume and their 

 enormous density. h is a remarkable fact that the 

 analysis founded on this impossible hypothesis perfectly 

 accounts for the anomaly proved to exist in the orbit of 

 Encke's comet, viz., its progressive acceleration. I fee! 

 bound to question this analysis, and to show (i) that its 

 primary basis is radically false, since it leads to the ad- 

 mission that a material and ponderable medium may- 

 remain immovable around the sun ; (2) that the conclu- 

 sion of this analysis, so far as it is valid and conformable 

 to observation, simply proves that there must exist an 

 action opposed to the movement of the comet and 

 directed along the tangent to its orbit. Various causes, 

 moreover, may lead to the same conclusion, and difter 

 only, as to other effects, in quantities difficult to appre- 

 ciate. But we learned above, from the phenomena of the 

 tails, that there also exists an action in the direction of 

 the radius vector. The resisting medium of Encke, or 

 the immense solar atmosphere of Newton, being physi- 

 cally impossible, I have been led, by two dilferent 

 ways, to a new force which would satisfy these data by 

 producing the two actions or components above men- 

 tioned : that which expels the cometary molecules in the 

 direction of the radius vector, and that which acts upon 

 the comet in the inverse ratio of its tangential velocity. 

 (To be continued.) 



