138 PROFESSOR PIAZZI SMYTH ON COMETARY PHYSICS. 
well marked changes or even characteristics at any part of their orbits, and are soon lost in the 
twilight even in their densest states ; there being little compression, and by axiom 4, little substance 
to compress, the mean distances being only 2°216, 3-502, and 3812, the earth’s distance being unity. 
In the case of Hatey’s comet, however, the appearances are very different, the excentricity being 
0-967, and the semiaxis major 17-988; hence, on this large body approaching the sun and under- 
going such a much more intense degree of compression, distinetly marked changes were seen almost 
from day to day, and at a certain distance from the perihelion it was of great brightness. But the 
perihelion distance being still large, about half that of the earth, or near fifty millions of miles, the 
condensation was not sufficient to enable the comet to be seen in moderate twilight, and hence it was 
not seen after the perihelion passage for more than two months, but then remained visible for nearly 
four months, so that it was lost sight of at about six months after perihelion passage. 
The great comet of 1844-5 had a less perihelion distance, viz., about 25 millions of miles, and 
a mean distance probably much greater, hence it was sufficiently concentrated in the neighbourhood of 
the sun to force itself on the notice of men within a week after perihelion: which implies a very much 
greater degree of brightness, than if Hanrry’s comet had been seen as early, when powerful telescopes, 
directed by means of an accurate ephemeris, were employed in the search. This comet remained in 
sight between three and four months, and when last seen was a faint nebulosity with little or no 
apparent concentration in any part. 
But the great comet of 1843 is again the decisive test, as this had a perihelion distance of only 
half a million miles, 60,000 only from the surface of the sun: here, therefore, we might expect to see 
the brightness excessive at and about the perihelion ; but the subsequent expansion, on account of 
the great mean distance, would be so rapid that the comet would be soon lost sight of by reason of 
faintness. Accordingly, we find that this comet pressed itself on men’s attention one day only after the 
perihelion passage ; and from its being so very bright then, and yet seen by so few, there can be little 
doubt but that it might have been observed the day before, if it had been looked for ; and would have 
been so seen, were not staring into the sun’s face and immediate vicinity rather a trying, and, conse- 
quently, an unpleasant occupation to most eyes, and seldom indulged in, especially in the warmer 
countries of the south, when the sun might have been that day unveiled from cloud, and was high in the 
sky. But, however, even the day after the perihelion passage, when the comet must have been much 
less dense than at that epoch, it was quite bright enough to be seen throughout the day within two 
degrees of the sun, and was then about one degree in apparent length; four days after it had increased 
to 25 degrees, in a fortnight to double that ; in a month it was so faint and distended as to be lost to 
most person’s eyes, and powerful telescopes only kept it in sight a few days longer. Its meteor-like 
brightness and short ephemeral existence were subjects of general remark in the south. 
This instance may be considered to settle the matter, but Mr Hrnn’s interesting comet of 1847 
as a later instance, and a well-marked one also, is very deserving of mention. He discovered this on 
February 6, 1847, as an exceedingly faint nebulous body approaching perihelion ; he observed the 
gradual condensation in the head and appearance of nucleus and tail, this last being about a degree 
long on March 9; and having computed the orbit and found the time of perihelion passage to be March 
30-269, Gr. M. T., and that the distance from the sun was then only four millions of miles, he called 
general attention to the circumstance under the hope that, 1st, the comet might be seen in daylight on 
that day ; and, 2d, that a long tail might be visible in the evening after sunset. In the former he was 
borne out by the fact, for he observed the comet himself with a refractor of 7 inches aperture at 11" 
A.M., within two degrees of the sun, and three other persons are recorded to have witnessed it too. I 
examined that part of the sky myself on the occasion, but with a telescope of only 3-7 inches aperture 
could see nothing : Mr Hinp himself found it a very difficult object to observe, so that the sizes of the 
two instruments may be taken as giving some measure of its visibility. In the latter supposition he 
was not confirmed; for no person saw a tail after sunset, and he himself says that the tail which he 
saw, exceedingly faint certainly, in the telescope in the day time, very nearly at the epoch of the peri- 
helion passage, was only 40” long,—but the 90th part of its length 21 days before. 
He was led to the first conclusion by the consideration, that the intensity of the light would vary 
as 

as (when r is the comet’s radius vector, and A its true distance from the earth), whence the 
comet should be at the time of perihelion 230 times brighter than that on March 8, when it was just 
perceptible to the naked eye. (Royal Astronomical Society’s Monthly Notices, vol. vii., p. 248.) But 
here it will be seen that with regard to the distance from the sun and perihelion, the intensity of solar 
illumination alone is taken account of; but the concentration of the comet at the perihelion must have 
greatly assisted the effect, and without this it seems pretty certain that the comet would not have 

