142 PROFESSOR PIAZZI SMYTH ON COMETARY PHYSICS. 
ing forth from one side only of the nucleus, that body should be driven far in the 
opposite direction: but by comparing its observed daily places during the peri- 
helion passages with the computed, we find that no deviations from any such 
anomalous causes are ever experienced. 
All these difficulties, however, vanish on considering the enaelaee of a comet 
to consist of separate molecules, each constituting an independent projectile, and 
bound together.only by their mutual gravitation and the laws mentioned above ; 
for then the size, character, and position of a comet being given at the perihelion, 
which we must look on as the normal state, all its principal variations of appear- 
ance during the rest of the orbit may be readily computed; and the return of 
every particle of the envelope to perihelion, or, vulgarly speaking, the retention of 
the tail by the nucleus, will be no more surprising, nor deviating from ordinary 
laws, than the return of the nucleus itself: certainly there is nothing “ in defiance 
of the laws of gravitation, and even of the received laws of motion,” as. stated 
by a supporter of the other theory to be the case with that. 
But following this principle further, we may expect that, in the multitude of 
molecules moving about amongst each other, occasional conglomerations may occur, 
after passing the proximity of some large planet, whose attraction acting much 
more strongly on one part of the envelope than the other, will so much alter the 
motion of the particles therein, that they will, after some revolutions, gradually 
collect together at a distance from the nucleus, and at length separate and become 
a distinct comet. Such a case having actually occurred under our own eyes, four 
years ago, with BieLa’s comet, when it was not at the time under the immediate 
influence of any planet in particular, nor in any trying part of its orbit round the 
sun, adds much additional weight to this view of the constitution of such bodies. 
This brings us to the second portion of the subject, viz., the corrections which 
should be applied to apparent observations to deduce the real phenomena. 
A comet being an elongated, gaseous, elastic, and semitransparent body, 
varying in size and density with its distance from the sun, evidently requires 
many different corrections, according to the point of view and the distance from 
which it is seen, to give an idea of its real nature at the instant of observation ; 
and needs other corrections, to reduce it from that instant to some other in which 
itis in a normal condition. This normal condition is plainly in perihelio (though 
a better general comparison of the volumes of different comets would be obtained 
by reducing each to its mean distance, as they would then be all of much more nearly 
equal density), and viewed at right angles to the line of its larger axis. This is a 
position which seems generally to have been taken for granted, though it never oc- 
curs even approximately. If we could see a comet at the instant of its perihelio, the 
plane of its orbit being inclined 90° to that of the ecliptic, and the radius vector 
being infinitely small, the above view would be nearly obtained, but would gradually 
