
PROFESSOR PIAZZI SMYTH ON COMETARY PHYSICS. 143 
be altered as the comet proceeded in its orbit. As seen from the sun, the comet 
should always present a circular body, and be equally illuminated on all sides, 
except in so far as the longer axis is inclined to the plane of the orbit: when the 
comet retreats towards the aphelion, the point of view from the earth becoming 
more nearly the same as that from the sun,—the comet should become rounder and 
rounder, as well as larger; and this is found actually to be the case,—the tails of 
HAu.ey’s and those of 1843 and 1844-5, having been, at and about the perihelion 
passage, narrow, and intense, and becoming at the last instant in which they were 
seen, wide, round, and diffuse. 
This, perhaps, together with the facts of phase and imperfect transparency 
(axioms 9 and 10), is sufficient to shew the importance of correcting the observa- 
tions for both the terrestrial and the solar elements of effect on the appearance 
of a comet, and be able to deduce its normal condition. That there may be other 
changes operating is very probable, but be that as it may, these effects of 
geometry, mechanics, and optics, actually exist to a very sensible amount, and 
their corrections must be applied before we can expect to discover any of the 
residual causes. 
T ought, doubtless, to apologize for having formed opinions different from Sir 
J. HerscHEt’s last, as he is confessedly the person, above all others, entitled to 
paramount weight in cometary physics; and it may be that I have not properly 
understood, and unintentionally have not here sufficiently represented the reasons 
upon which his old conclusions have been altered, and on which he has thought 
it allowable to introduce electrical and other forces, to explain phenomena 
amongst the celestial bodies; and I would therefore refer inquirers to his works 
themselves. But while on the one side, I hope that what has been here brought for- 
_ ward with regard to the complete body of a comet and its symmetrical and geome- 
trical form, when freed of the effects of phase, may remove one of the objections 
which he felt to allowing the all-sufficiency of gravity acting on a group of inde- 
pendent molecules ; on the other side, I not only allow, but think it extremely pro- 
bable, that some other effects besides those already mentioned, may legitimately 
occur; and if heat and rotation on the earth produce our trade-winds and hur- 
ricanes, much greater effects may follow the more violent alterations of tempera- 
ture and velocity of motion in a comet. Further, as confirming a curious feature 
noticed in HALLEY’s comet, by Sir Joun, after the perihelion passage, viz., a long 
thin ray posterior to the nucleus, to which it might perform the office, he suggest- 
ed, of conveying back the vapour driven off in front at perihelion; I would men- 
tion, that a ray of the same sort was seen posterior to the nucleus of the great 
comet of 1843, of extravagant length and excessive thinness, appearing as a very 
fine line of light, and traceable for many degrees up the tail: in these particu- 
lars, bearing some relation, perhaps, to the excentricity of the orbit, and to the 
VOL. XX. PART I. 2Q 
