140 PROFESSOR PIAZZI SMYTH ON COMETARY PHYSICS. 
owing to the probable minuteness of its mass. Cohesion can hardly be supposed 
to exist in a gaseous or nebulous body of such tenuity; so that the only bond of 
union between its molecules must be their fecble gravitation to each other, which 
is hardly more than mere juxtaposition in space. Hence we must regard each 
molecule as constituting almost a separate, independent projectile, describing its 
own parabola about the sun. Now, the interval between two or more parabolas 
described about a common focus, and having their axes coincident, is a minimum 
at the perihelion, and increases as we recede from it” in the sesquiplicate ratio of 
the radius vector. The obervations of EncKr’s comet, which Sir Joun treated by 
this theory, shewed rather a more rapid rate of increase and decrease; which 
might, he thought, be readily accounted for by the effect of the brighter back- 
ground of sky on which the comet was projected as it approached its perihelion, 
and vice versd. But whether any other forces may have part in the entire pheno- 
menon presented, he concludes that the property above pointed out, cannot but 
be allowed to be a vera causa, and to have some share in the production of the 
effect. 
To the latter part of this opinion every one must assent; and with respect 
to the want of agreement between theory and observation, in the case quoted by 
Sir Jon of a small comet, in addition to the observations themselves requiring cor- 
rection, for the cause he has mentioned, the theoretical quantity requires it also on 
account of the greater attraction of the molecules upon each other at the perihelion, 
by reason of their increased proximity; while, moreover, the figure of the comet, and 
the direction in which it is seen, require also to be taken into consideration. With 
regard to /arge comets, which seem generally to have been thought to be under the 
dominion of absolutely different laws, the decrease and concentration of the tail at 
perihelio, is fully accounted for by this 12th axiom, as well as some other pheno- 
mena, the perplexing nature of which, when viewed by the light of any other theory, 
may be gathered by the account given by Sir J. Herscuen himself, at the conclusion 
of the chapter on comets in his work of last year (Outlines of Astronomy.) 
“Tt is in a physical point of view that these bodies offer the greatest stimulus 
to our curiosity. There is, beyond question, some profound secret and mystery 
concerned in the phenomena of their tails. Perhaps it is not too much to hope 
that future observations, borrowing every aid from rational speculation, grounded 
on the progress of physical science generally (especially those branches of it 
which relate to the ethereal or imponderable elements), may ere long enable us 
to penetrate this mystery, and to declare whether it is really matter, in the ordi- 
nary acceptation of the term, which is projected from their heads with such extra- 
vagant velocity, and if not impelled, at least directed, in its course by a reference 
to the sun, as its point of avoidance. In no respect is the question as to the 
materiality of the tail more forcibly pressed on us for consideration, than in that 
of the enormous sweep that it makes round the sun in perihelio, in the manner of 
et! ee a ee 
