IN THE SOLAR SYSTEM. 



87 



' 



too partial, has given an incorrect version of Sir Isaac Newton's views upon the subject of 

 comets' tails.* 



We may easily pardon in the author of the corpuscular theory of light, his qualified 

 indorsement of Kepler's theory of comets' tails. The true views of Newton upon this 

 subject were undoubtedly those described in the last part of the quotation from the Prin- 

 cipia. 



It is certainly a subject entitled to consideration, why so ingenious and plausible a theory 

 should have met with such slender success. There are probably several reasons. Firstly, 

 the existence of any fluid in the interplanetary spaces, was all conjecture. Secondly, 

 those men who believed in the existence of an ether, had their minds so imbued with the 

 notions of the ancients upon the subject, that they could not admit its being possessed of 

 sufficient materiality to direct the tail of a comet. Lastly, the minds of the astronomers 

 of the present century have probably been confirmed in their opposition to the Newtonian 

 theory, by the appearance of the comet of 1823, and of others, with their tails directed 



towards the sun. 



My own belief is, that the usual direction of the tails of comets is occasioned by a cause 

 similar to that which gives to the flame of a candle a direction away from the earth, viz., 

 that the specific gravity of the matter of the tail is less than that of the surrounding ether, 

 and thus, by external pressure, the heated mass is forced away from the centre of attraction. 



I hope, by the aid of discoveries made since Newton's day, to explain satisfactorily the 

 cause of a different direction in the few exceptional cases which have been observed. 



The next phenomenon of comets I have to mention, which has been ascribed to the 

 influence of ether, is the one upon which this essay is founded, — the discovery of Professor 



Encke. 



" On comparing the intervals between the successive perihelion passages of Encke's 

 comet, after allowing, in the most careful and exact manner, for all the disturbances due 

 to the action of the planets, a very singular fact has come to light, viz., that the periods 

 are continually diminishing, or, in other words, the mean distance from the sun, or the 

 major axis of the ellipse, dwindling by slow and regular degrees, at the rate of about 0.11 

 days per revolution. This is evidently the effect which would be produced by resistance 

 experienced by the comet from a very rare ethereal medium pervading the regions in which 

 it moves ; for such resistance, by diminishing its actual velocity, would diminish also its 

 centrifugal force, and thus give the sun more power to draw it nearer. Accordingly, this 

 is the solution proposed by Encke, and at present generally received."! 



This point in the history of comets is of more importance than any other connected 

 with the subject, as all deductions concerning the existence of ether are based upon it. 



* Astronomic Populaire, vol. ii, p. 411. f Heraehel's Outlines, chapter xi. 



