
PROFESSOR PIAZZI SMYTH ON COMETARY PHYSICS. 137 
whole body of the comet was seen, but what his idea of the true complete form of its body was, he 
does not give; but, as he states, that a slight change of brightness near the head was “ the only 
thing to distinguish the nucleus from the tail at that point ;” and, further, “ that every part of the 
tail was as well defined as the moon on a clear day ;” it would appear to be quite safe to infer, that 
the tail was not forked; and that the base, instead of a broad or forked, or many-pointed indistinct 
termination, was as well and sharply outlined as the limbs. A notable distinction this to every 
subsequent view obtained on succeeding days; and, indeed, in the case of every other comet what- 
ever observed at a great distance from the sun,—when, whatever the definition of the limbs of the 
tail, the termination or the base has always been so excessively uncertain, that different persons have 
varied several degrees in assigning the place of it. 
Alas! indeed, that the practical astronomy of the present day did not take better account of 
this unique and critical instance which was offered by the skies of our times; centuries may elapse 
before another such instance may occur. and this question of the real and complete form of a comet 
may be in abeyance as long. Something may, doubtless, be done by rigid examination of all the 
persons who did witness the phenomenon in the comparatively imperfect form of the day after the 
perihelion passage; but their answers would not be very safe now, so many years after the event, 
and after the promulgation of a particular theory. Something might also, perhaps, be done, by 
careful and photometrical observation of the faintest nebulz, while the darker part of a comet’s 
tail, if it exists, must be passing across them. But this is a very unpromising method, for comets, 
at all periods, attenuated, become so exceedingly diffuse by the time that they have reached a 
sufficient distance from the sun, to be viewed for any length of time in a dark sky, and contrasted 
therein with very faint nebule,—that we can hardly expect to obtain any certain indication in this 
manner. The only sure way is for the comet to be so very close to the sun, that rays from some 
part or other of his surface will reach every portion of the body of the comet directly, i. ¢., without 
haying to pass through any other part in order to arrive there. 
The fact of this great and invaluable opportunity having been lost, would seem to shew that it 
is highly desirable that extra meridian observations should be made and watched for by some public 
observatory in its official routine, instead of being abandoned altogether to amateurs. It is high time 
that our observatories should be placed in the clearer climates of some of the colonies, and that the 
most favourable geographical positions should be sought for, rather than the most convenient places in 
a social point of view ; for this results in smoky towns in our own beclouded country being selected as 
the places where the stars are if possible to be observed. 
10. The gaseous envelope is of extreme tenuity, is elastic, and with regard 
to light is slightly reflective and imperfectly transparent ; it decreases in size, but 
increases in density, and light reflective power in approaching the perihelion, and 
the reverse when receding from it; and this occurs in a degree proportioned to 
the excentricity of the orbits of the comets. 
(10.) That the gaseous envelope of a comet is of extreme tenuity, and is elastic, slightly reflec- 
tive and imperfectly transparent, is apparently confessed on all hands, and is proved by the pheno- 
mena presented by every comet. That it increases in density and light-reflective power with its 
proximity to the perihelion, and that this occurs in a degree proportioned to the excentricity of the 
orbit, requires, at least the latter part does, that the instances on which it is founded should be men- 
tioned; for though the contraction in size of small comets on approaching the sun had been remarked, 
yet some had maintained it to be accompanied by a decrease in density, by an actual evaporation and 
disappearance at perihelio ; and no one that I am acquainted with had applied it to the larger comets 
also, or compared the degree of it, with the excentricity of the orbit. 
With regard to the effect of excentricity of orbit, a small proportion of it should make a comet 
visible for a long period on either side of the perihelion, from the lesser degree of attenuation and expan- 
sion of its substance at a distance therefrom ; and it should also be lost in the sun’s rays for a consi- 
derable time at and about the perihelion passage, from the matter never being compressed into a sufhi- 
ciently dense body to be visible in the blaze of day. This rule appears well borne out by both small 
and large comets ; the small ones, for instance Encke’s, Brena’s, and Fayve’s, which have for the ratio 
of the excentricity to the semiaxis major, the numbers respectively, 0-847, 0-755, 0-555, shew no very 
