132 PROFESSOR PIAZZI SMYTH ON COMETARY PHYSICS. 
comets were seen chiefly at or about the perihelion passages, they were said to 
be produced then; to have been shot out, and then drawn in again, or dissipated ; 
and numerous have been the theories to explain this creation and extinction. 
And yet of all the facts that have been ascertained, if any of them can be so con- 
sidered, with regard to the physical appearances of comets, of none may we be 
more sure than that the tails of comets, in place of being /argest, or existing only 
at the perihelion point of the orbits, are then the smallest. Comets of every size, 
(the distinction of those said to be with and without tails is visionary, or rather 
the tail is equally a part of the general body of the comet, as the so-called head, 
and obeys the same laws), when accurately observed, have always been found to 
decrease in coming to perihelion, and to increase in size in retreating therefrom ; 
this condensation of substance, producing more power to reflect light at that 
period of the orbit, when, from the closer proximity to the sun, there is more 
light to reflect. These two causes combining, and both increasing most rapidly 
with comets of great excentricity and small perihelion distance, occasions the 
sight, all of a sudden, of a long cometric ray in our skies, when the previous 
night, or at least the previous clear night, there was none bright enough to catch 
men’s eyes. As the comet leaves the sun, the tail or body expands, and partly 
from its consequent greater rarity, and the diminishing intensity of its solar illu- 
mination, is lost to our sight; and only the denser roundish portion about the 
head remains visible. This is likewise expanding, and is at length also lost sight 
of for the same reason. In like manner a comet reappears, first the oval mass 
about the head, and then the tail gradually strengthens; but its aspect will 
materially depend not only on its distance from the sun, but on our distance from 
it, and the direction of our line of sight with the longer axis of the body. 
Having had the good fortune to see a rather large number of comets, both 
great and small, and under circumstances favourable above the average, I hope 
that I may be of some service to theorists, by stating what data may be looked 
upon as well fixed with regard to these phenomena; by pointing out some cor- 
rections which are absolutely necessary to be made upon the observations, before 
any good and safe grounds for theorizing can be procured, but which corrections 
never have been made; and by pointing out the most probable method of im- 
proving the observations themselves, which, as at present conducted, are by no 
means satisfactory. 
With regard then to the physical nature of comets, we may take the follow- 
ing as axioms :— 
1. A comet consists of a nucleus, and one or more gaseous envelopes. 
(1.) No instance has ever been recorded, at least since the fabulous days of astronomy, of a 
comet having ever been seen without some gaseous appendage, forming, indeed, a distinctive feature 
at a distance from every other body of the solar system; at a distance, because very close to one of 
the planets, especially one or two of the asteroids, something of its atmosphere might be ob- 

