
PROFESSOR PIAZZI SMYTH ON COMETARY PHYSICS. 133 
served; but, at the actual distances at which they are viewed, the most powerful telescopes never 
show these atmospheres in the same manner as those of a comet; they are indicated only by a very 
different order of phenomena. 
When Uranus was first discovered, and no one dreamt of planets beyond Saturn, it was called 
a comet; not because its form was like that of any recognised comet, but because it was expected 
that its orbit would prove similar; when, however, the real nature of its path was discovered, the 
appellation of comet was quickly retracted. : 
So much for the necessity of a gaseous envelope; of the equal importance of a nucleus, it may 
be remarked, that although some comets are described as having nuclei, and others as having none ; 
this turns out to be but negative testimony, inasmuch as these latter bodies have always been the 
fainter, smaller, and more distant ones, in which the nucleus should have been so much the more 
difficult to distinguish; and if it has not been actually observed itself, there has at least been in- 
_ variably noticed in every recorded comet, some one point where the gaseous matter was visibly more 
concentrated than in other parts, indicating thus a virtual or a dark nucleus, if not an actual and 
a reflective one: while observation, combined with calculation, has satisfactorily shewn, that in 
comets of every degree of size and excentricity, the mass is so very nearly concentrated in this 
nucleoid centre, that that need alone be referred to in all determinations of the orbit. 
2. The nucleus if solid and material, is exceedingly small. 
(2.) Every advance of our knowledge has tended to diminish the possible size of the solid nuclei 
of comets, planetary perturbation has shewn them to have no sensible mass, and telescopic observa- 
tion no sensible size; and in the cases of comets of all sizes, observers have witnessed them pass 
over stars in every position, except, perhaps, exactly centrically with the nucleus, without perceiving 
any obscuration of the stellar rays. 
The old observers have certainly spoken of very large nuclei, but they evidently meant rather 
the head, which, in some comets at certain parts of the orbit, presents in small telescopes an ap- 
pearance of planetary opacity and definition. 
Such was the case with the great comet of 1843, for three or four days after having passed its 
perihelion ; in small telescopes it was difficult to avoid believing in the existence of an actual planetary 
nucleus of very notable size; but the fourteen feet reflector of the Cape Observatory shewed the 
borders of this head to be filmy, and exhibited small stars shining through it ; day after day it expanded 
and became less defined, until at last it ceased to present a solid appearance in any telescope; and at 
no time was there anything larger than a stellar point, to which the attribute of hard or heavy matter 
might be expected to apply. 
3. The nucleus is excentrically situated in the gaseous body. 
(3.) The nucleus actual or virtual, has never been observed in the middle of the envelope, but 
always nearer one end than the other, the envelopes too, never being round, but invariably more or 
less elongated. 
4. Comets of longest period have the largest bodies. 
(4.) This is the general result of cometary statistics, but need not be any more strictly true, than 
that the largest planets are all at the greatest distances from the sun ; they are not strictly ranged in the 
order of distance agreeably with size, but as a general rule merely, the smaller are closer to the sun 
than the larger planets. In the same way the telescopic comets, when sufficiently numerous obser- 
vations have been obtained, have almost always been found to have short periods, and those very 
brilliant ones, with not only long but broad and dense tails, have invariably been found to be of long 
period. 
5. Those comets whose orbits have the greatest excentricity, are the most 
excentrically situated in their envelopes, or, vulgarly speaking, have the longest 
tails. 
