AO On the Tails of Comets. F 
greater or less in proportion as the union of rays is more or less _ a 
distant from the comet. It is manifest that if a ray of lightcould’ 
be traced during its entire course from the sun to a planet, it would 
present a similar phenomenon, equal in degree if the motion of — 
the planet were swift as that of acomet. The comets of Biela 4 
and Encke have no tails, nor is there, strictly speaking, a nucleus * 
in either. That of Encke, during the long period in 1828, when _ 
its position was so favorable to observation, had the appearance of 
a mere film of vapor, nearly circular but not well defined, and no 
central, stellar point could be detected with the telescopic power — 
which T employed on that occasion. In fact, all the phenomena 
of the tails of comets appear to be so well clea by this theory 
that I can not doubt its truth, although nothing like demonstration 
accompanies it. There are indeed optical difficulties which I 
have been unable to overcome: no one however which may not 
be fairly attributed to our ignorance of the particular physical 
constitution of these bodies. It is no small confirmation of the 
truth of this explanation of the tails of comets, that there is not 
the slightest evidence, worthy of confidence, that the earth which 
we inhabit has ever been sensibly affected by a visitation from 
these enormous appendages, while the chance of collision between _ 
the earth and the nucleus of a comet, properly so ealled, is ex- 
ceedingly small; yet when we reflect upon the number of comets 
belonging to our system, the hundreds that range within the 
earth’s orbit, that their paths have every possible inclination to 
the ecliptic, that these immensely extended trains, projected in 
a direction from the sun, describe an inconceivable sweep when | 
they are encompassing the sun in the region of their perihelion ;— . 
I say in view of these circumstances, it is difficult to avoid the 
conjecture, nay, it is exceedingly probable that these appendages, 
in very many instances, have brushed across the surface of our 
planet, harmlessly and unperceived. 
I submit this theory (if indeed it is entitled to that name) to 
the consideration of the scientific, having no point to gain, 00 
wish to gratify, but the promotion of science and the progress of 
truth, and if insuperable objections to it are raised, and my rea 
soning should prove fallacious, there will be at least one valuable — 
result, that of showing what the tails of comets are not; more- 
over, it —_ be the humble means of exciting further inquiry 02 
this inte: topic. 
Nantucket, 10th mo., Ist, 1839. 
Bits ae 
a 
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