On the Tails of Comets. ' 39 



observation on the period of the Eiicke comet, states unhesitating- 

 ly that the cause " can be nothing but the resistance of the ether." 

 And Dr. Bowditch, distinguished as he was for cautiousness, ful- 

 ly recognized the effect of an ethereal medium, in the translation 

 of the Mecanique Celeste. The fact however that Halley's 

 comet at its late return reached its perihelion later rather than 

 earlier than the calculated time, independent of an allowance for 

 a resisting medium, seems to have created some doubts in refer- 

 ence to the doctrine of resistance ; but of the three comets whose 

 periods are certainly known, those of Biela and Encke only can 

 be relied on as indicating resistance, inasmuch as that of Halley 

 has its aphelion in a region beyond the scan of human power, and 

 the influence of planetary bodies which may exist there, is now 

 and will perhaps forever remain unknown to us. These facts 

 then, and the concurring opinions of the high authority above 

 quoted, render it nearly unquestionable that there is diffused 

 through the celestial regions an ethereal and exceedingly elastic 

 medium ; nor would it be unreasonable to suppose that this very 

 medium constitutes the solar atmosphere, of which the zodiacal 

 light may be a denser region. 



When an opportunity is offered to observe a comet remote from 

 the sun, it is generally found to be unaccompanied with a tail ; 

 but as it approaches, the tail begins to appear, and its length and 

 brilliancy increase, till it reaches the perihelion of its orbit, and 

 by an illusion, sometimes beyond tliis point. Although there is 

 some degree of diversity in the form of the tails of different com- 

 ets, yet they generally consist of two streams of light, not abso- 

 lutely distinct from each other. In other words, the borders of the 

 tail are brightest, plainly indicating a hoUowness, the line of vis- 

 ion necessarily meeting with a greater number of luminous points 

 on the edges than through the middle. Can any explanation of 

 this hoUowness be given, more simple and philosophical, than that 

 the rays of the sun^s light are more obstructed by the denser than 

 the rarer portions of the comet ? 



That there is, in these tails, which acquire a considerable length, 

 a slight curve, concave to that portion of the orbit which the comet 

 has left, there is ample testimony. Now as light is progressive, 

 a portion of time must elapse while the rays of light are passing 

 from the head of the comet to their point of union, and during 

 this period the comet moves onward in its course, and the result 

 necessarily is a gentle or slight curve in the tail, the effect being 



