134 PROFESSOR PIAZZI SMYTH ON COMETARY PHYSICS. 
(5.) This may not be strictly true, but yet is assuredly a very marked feature in the statistics of 
the question. The great comet of 1843, whose orbit was the most excentrie ever known, i. e., had 
the least perihelion, but great aphelion, distance,—had also the longest and narrowest tail, and the 
smallest head ; consequently the nucleus situated near the centre of the latter was most excentrically 
situated in the gaseous envelope. Haxrey’s comet, and that of 1811, of less excentricity of orbit, 
had shorter and broader tails and larger heads; and their nuclei, consequently, less excentrie: while 
the telescopic comets of short period, and aphelion not extraordinarily greater than their perihelion 
distances, exhibit merely somewhat oval masses of vapour. 
6. A comet revolves on ap axis passing through the nucleus, and at right 
angles to the major axis of the envelope, in the same period of time that it takes 
to revolve about the sun: hence, the tail being turned away from the sun in the 
normal position, is turned away from him in all other parts of the orbit also. 
(6.) Every comet has invariably been observed to have its tail turned away from the sun in 
every part of its orbit; this was the first notable fact established in cometary physics, and the axiom 
is but a different statement of it. 
7. This axis is not at right angles to the plane of the orbit, but variously in- 
clined in the case of different comets, as with the planets. 
(7.) There is no reason to expect the contrary; indeed, analogy rather leads us to this conclusion, 
and it may, if admitted, be sufficient to explain the apparent want of symmetry observed in the tail 
of Haxtey’s comet, that of 1819, and most, if not all, which have been the subject of special atten- 
tion ; and it may tend to account for some of the differences in the appearance of the former body in 
approaching and leaving its perihelion, at considerable but equal distances on either side of that point. 
8. A quicker rotation round the longer axis of the body also appears to 
exist. 
(8.) This seemed to be almost proved by some of the changes which took place in the head of 
the great comet of 1843, night after night, in the earlier part of its apparition ; for instance, a double- 
winged head, laterally, one night, becoming a single and centrically winged, or rather a tailed-head 
the next night ; but when a body is seen for so very short a space of time, for a few minutes only in 
twenty-four hours ; and sometimes, perhaps, for several days, even that short glimpse is prevented 
by clouds,—it becomes extremely difficult to separate in such a body as a comet, in which there is 
nothing decided and tangible, and fixed either in size or brightness, any indications of revolution 
from those of the other motions and changes which are going on simultaneously. But it seems a 
point well worthy of attention, and to be proved or disproved. 
9. A comet shines by reflected light, and shews a sensible phase ; the quan- 
tity, form, and position, therefore, of its component matter, cannot be judged of 
by the eye alone. 
(9.) That comets shine by reflected light, is considered to have been proved by Araco’s polarizing 
experiment ; and was inferred before by every analogy in the planetary system; but all appearance 
of phase has been denied, this, therefore, requires a little explanation. The supposed absence of phase 
has been attributed to the excessive tenuity of the matter of the comet, and the case has been illustrated 
by reference to the thin clouds often seen in the west after sunset, or in the east before sunrise, 
glowing in the solar rays, literally drenched with light, and exhibiting no distinction of light and 
dark side, A little examination of this instance would have shewn that the conclusion is not so 
safe; the whole of the cloud being so bright, the difference of illumination of the two sides of it is 

