+1OU9. 4 
glimpses; but the star6 retains its former 
hight. 
Nebulous Appearance of the Conet. 
Dec. 6. The head of the comet, view- 
ed with a mirror of twenty: four inches eli- 
ameter, resembles now. one of those nebu- 
he, oii | in my catalogues would have 
been described, “a very large, brilliant, 
round nebula, suddenly ‘much brighter In 
the middle.” 
Dec. 16.’ Seven-feet refiector. The 
night being fine, and the moon not risen, 
the comet resembles a- very bright, large, 
irregular, round nebula, very “gradually 
much brighter in the aaule. with a faint 
nebulosity on the south-preceding side. 
Jan. 1, 1808. Seven-feet. “ Very 
bright, very large, very gradually much 
brighter in the middle.” 
If I had not known this'to be a comet, 
I should have added to my description of 
Atas a nebula, that the centre of it might 
consist of very small stars; but this being 
impossible, I directed my ten-teet tele- 
scope with a high power. to the comet, in 
order to ascertain the cause of this ap- 
pearance ; in consequence of which f per- 
ceived several small stars shining through 
the nebulosity of the coma. 
Jan. 14. Sevenfeét. ‘ Bright, pretty 
large, irregular round, brighter in the mid- 
ale.” 
Feb. 2. Ten-feet, twenty-four-inch 
aperture. “< Very bright, lange, irregular 
round, very gradual ly oe brighter in 
the middle.” There is a faint difused ne- 
bulosity on the north preceding side; I 
take it to be the vanishing remains of the 
comet’s tail. 
Feb. 19. Considerably bright; about 
3th of the field = 3'.267 fin diameter, 
gradually brighter. in. the ouiddle.” The 
. faint nebulosityin the place where the rail 
used to be, -still.projects-a little further 
from the centre than in other directions. 
Feb. 21. Less bright than on the. 19th; 
nearly of the same size : gradually br ishter 
it the middle. The nebulosity stula tittle 
projectiag on the side where ihe tail-used 
to be. 
Result of the foregoing, Observations. 
From the observations which-are iow 
before us, we may draw.some inferences, 
which. will be of considerable i impurtane e 
with regard to the information they give 
us, not ‘only of the size of the comet, “but 
also of the nature of its lilumination,. 
A visible, round, and weil defined disk, 
shining in every part of it with equal bright 
Ness, elucidates two material circumstan- 
ces ; for since the nucleus.of this comet, 
like the body of a planet, appeared in the 
-— 
£1 seca rite a GC} felts Hew GUCECLICS 107 
shape of a disk, which was experimentally 
found to be areal one, we have.good rea- 
son to believe that it ‘consists of seme con- 
densed or solid body, the magnitude of 
which may be ascertained by calculation. 
For instance, we have seen, that.its ; appa- 
rent diameter, the 19th oF Gctober, Gh. 
20', was uot quite.so large.as that of .the 
third satellite of Jupiter. In erder there 
tore to have seme idea of;the real magni- 
tude ef aur,comer, we may-admit that.its 
diameter at the time of obsersation was 
about 1”, which certainly cannot be far 
from biedy: The diameter of the third sa 
tellite of Jupiter, however, is. known to 
have a permanent disk, suc h as may at any 
‘convenient time be measured withall the 
accuracy that can be ased ;. and, when the 
result of such a measure has given us the 
“diameter of this satellite, it may by calcu- 
lation be brought to Ad aicwanee from the 
earth at which, in my observation, it was 
compared withahe diameierof the comet, 
and thus more accuracy, if it should be 
required, may be obtained. ‘The follow- 
Ing result of my ea! lculation, however, ap- 
ears to me quite Sta ee for the. pur 
pose of general information. From‘ the 
per ihelion distance 0'647491, and the rest 
of the given elements of the comet, we 
find, that its distance from the. ascending 
node on its ola at.the time of observa- 
tien was 73° 45/ 44” ; and having also the 
earth’s distance from the same node, and 
the inclination of the comet’s orbit, we 
coi wpute by these data the. angle at the 
sun, Phen, by calculating in the next 
place the radius vector of the comet, and 
having likewise the distanee of the earth 
trom the sun, we find by computation, 
that the distance of the comet from the 
earth dt the time of cohservation was 
1169192, the mean distance of the 
earth beine 1. Now since the. disk of 
the comet was ahserved,to subtend an,an-~ 
gle of 1", which brgught to the mean dis- 
tance of the earth gives 1/-169, and since 
we also, hnow that the earth's diameter, 
which, according to Mr. Dalby, is 7913-2 
mules*, sabtands at the same distance an 
anule of 17-2, we deduce from these prin- 
ciples the edi diameter of the comet, 
LW hich 1s. 538 miles. 
Havi lng thus inv estigated the magnitude 
of our comet, we may in the next place 
also apply calculation to its illumination, 
The observations, relating to the light of 
*, See Bbiigsop bicals Trageactions for 1791, 
p- 239. Mr Dalby gives the two semiaxes of 
the Earth, from: a mean of which the above 
diameter 7913: 1682 is sical 
bi : the x 
a 
