iy > 
4 
b, 

PROFESSOR PIAZZI SMYTH ON COMETARY PHYSICS. 139 
been visible : then the introduction of the distance from the earth does not appear correct, for although 
this may change the apparent diameter of the body, it does not at all alter the intrinsic brightness of the 
surface. 
The second conclusion he was led to, by the old erroneous idea (to use his own words) that ‘ the 
close approach of the comet to the sun would be likely to produce a tail of considerable length :” 
but in place of so doing it was contracted in size to a very small compass. This additional instance 
of the prevalence of an idea so completely the reverse of the fact, will, I hope, excuse me from having 
attempted in so very crude a manner to establish what appears to me the grand statistical truths of 
cometary physics. But if I have not been able to agree with Mr Hivp im his physical ideas, I must 
express my testimony of his high standing in the more important question of the motions of comets ; 
here he has indeed filled an honourable niche, which had been long, if not always, unfilled in the 
cometary credit and fame of this country. 
A general result in cometography, certainly following the establishment of this axiom, is, that 
when the length of the tail of any comet of celebrity is described in millions of miles, a very favourite 
method with most writers, it will be absolutely necessary to accompany it with an account of the 
part of its orbit, where the comet is supposed to have been at the time: without this, the statement 
of an actual length, is as absurd as the fixation of the place of the magnetic pole, without a date 
being attached, ; 
11. The axis of the tail of a comet is straight at the perihelion, but at any 
point between this and the aphelion is curved, and is concave toward the latter, 
the radius of curvature being inversely as the excentricity. 
(11.) This I will not attempt to lay much stress upon ; but certainly the tails of the comets of 
Hatxey, of 1843, and of 1844-5, were sensibly straight near the perihelion ; and the two latter 
became curved after it, the former more than the latter, and they were concave to the direction im 
which they wete proceeding ; precisely the reverse of the general belief, which states them to bend 
backwards at the extremity of the tail, as if experiencing some resistance, when whirled round the 
perihelion with such exceeding velocity. 
The direction in which those two comets were proceeding at the time was towards the aphelion ; 
and I have not had any opportunity of examining a large comet coming up to the perihelion. The 
great comet of 1843 would have been sufficient to settle the question, but I have only heard of one 
person (a Commissariat officer voyaging from New South Wales to the Cape), who saw it in the 
eastern skies before sunrise and the perihelion passage ; and he had made no observations. 
12. The molecules composing the envelope of a comet are only held together 
by their mutual gravitation, each constituting almost a separate projectile, and 
describing its own parabola about the sun. 

The 12th axiom is Sir Jon Herscner’s, and taken in conjunction with the 
others, seems generally to explain all the principal variations in appearance, and 
affords ground for testing each exactly by calculation, and thereby of ascertain- 
ing what residual phenomena may be due to laws others than those of gravita- 
tion, mechanics, and optics. 
After alluding to the observed concentration of EncKE’s comet near perihelio, 
and the error of attempting to account for it by the pressure of a supposed ether 
in the vicinity of the sun, Sir Joun says (Royal Astronomical Soctety’s Memoirs, 
vol. vi.), “ It appears to me that the phenomenon is (if not wholly, at least par- 
tially) explicable on a much less gratuitous supposition, viz., that of the extremely 
feeble attractive force by which the matter of a comet must be held together, 
VOL. XX. PART I. 2P 
