On the Tails f Comet 39 
_ observation on the period ‘abthe Encke 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 Mécanique Céleste. The fact however that Halley’s 
comet at its late return reached its perihelion Jager 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 
5 ___ 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 
7 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 conssisabeh the solar atmosphere, of which the zodiacal 
e legit may be a denser region. 
. », - ~ When an opportunity is offered to sania a comet remote from 
Es iit, 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 
._ byan illusion, sometimes beyond this point. Although there is 
degree of diversity in the form of the tails of different com- 
ets, yet they generally consist of two streams of light, not abso- 
lately, distinct from each other. In other words, the borders of the 
‘tal are brightest, plainly indicating a hollowness, the line of vis- 
-- lon necessarily meeting with a greater number of luminous points 
on the edges than through the middle. Can any explanation of 
: this hollowness 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 ectsidua ae. 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, 
j 
~. _* @ 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 wants 
necessarily is a a gentle or slight curve in the tail, the effect being 
