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Hu Oct. 28. My large ten-lcet telescope, with the mirror of twenty-lbur inches in diameter, does nut increase the size ofthe nucleus. , Oct. 6. Being fully aware ol' the objections that may be nude aguinsttlie m¢_ thod' of com aringythe magnitude ul' the nucleus of tlie comet yvith ohjects :lint cannot be seen together, I had recourse to the satellites of Ju iser for n more decisive result, and withmy seven-l1>ut telescope, power 202, I viewecl tlie disk of the third satellite and ol' the nucleus of the comet nltcrnately. They were both already too low to he seen very distinctly; the diameter of the nucleus however nppenred to be less than twice that of the satellite. 6, , Oct. 18. With £8 ten-feet reflector, and the power 221~,|;$in\ilar estimation was bu lligight of the moon ,§i)'comparison. ii0ct 519 cymrepared a new tenfeét mirror, the delicate polish of my Former one having sulierell a little Front heing expoaul to damp nil' in n :cturnul oh lervations 'lliis new one being uncomf manly distinct, and the uirulso remurknbly clw,I. turned the telescope from the . ' .mthird sdtelhm; andsuw -distinctly larger tltnn Ml.; \ E
ggined, because the exertion of a power
Comparing the nucleus also with the
as I saw is
les sntel lite.
tbesraltefnste found ~ some beautifully ~ s
the moon hos not yet risen to interfere with the light of the comet.
Nov. 20. With a seven-feet reflector and power only 75,1 can also see the nucleus; it is extremely smali, being little mure than a mere point.
Of the Head of the Comet. When the comet is viewed with an inferior telescope, or it* the magnifying oower, with a pretty good one, is cither much too low, or much too high, the very bright rays immediately contiguous to the nucleus will seem to belong to it, and form what may be called the head.
Oct. 19. I examined (he head of the cornet with an indifferent telescope, in the manner I have described, and found it apparently of the size of the planet Jupiter, when it is viewed with the same telescope and magnifyin;; power.
With a good telescope, I saw in the centre of the head a very snuill well-defined round point.
Nov. 20. The head of the comet is now less brilliant than it has been. Of the Coma of the Comtt. Tlie coma is the nebulous appearance ■oirounding the head.
Oct. 13. Bv the field of view of my sefleetor, I estimate the coma of the comet to be about six minutes in diameter. Dec. 6. The extent of the coma, with a mirror of twenty-four inches diameteris now about 4.45.
Of the Tail of the Comet. Oct. lb. 7h. Willi a night glass, which has a field of view of nearly 5°, I estimated the length of the tail to be 3°"-; but twilight is still very strong, which may prevent my seeing the whole of it.
Nov. 2<x The tail of the comet is still of aconsiderahle length, certainly not less than a J degrees.
Oct. 26. The tail of the comet is considerably lohger on the south-preceding, than on the north-following side.
It is not bifid, as I have seen the comet of 176!) delineated by a gentleman who had carefully observed it*.
Oct. 28. Seven-feet reflector. The south-preceding side of the tail in nil its length, except towards the end, is very well defined; but the north-following side is every where hazy and irregular, especially towards the end; it is also shorter than the south-preceding one.
The shape of the unequal lentjtli of the titles of the tail, when nttentivelyvieived, is visible in a night glass, and even to the raked eye.
* Or. Lisd of Winder.
Oct. SI. Ten-feet reflector. The tail continues to be better defined on the south-preceding than on the north-following side.
Dec. 6. The length of the tail is now reduced to about 93' of a degree. Of' the Denst't/ >>f the Ciuua and Tail ef the Comet. Many authors have said, that the tails of comets are of so rare a texture, as not to art'ect the light of the smallest stars that are seen through them. Unwilling to take any thing upon trust, that may be brought to the test of observation, 1 took notice of many small stars, that were occasionly covered by the coma and the tail, and the result is as follows.
Oct. 26. 6h. 15'. Large ten-feet reflector, twenty-four inch.es aperture. A small star within the coma is equally faint with two other staro that are ou the northfollowing side of ti.e comet, but without the coma.
7h. SO*. The comr. being partly removed from the siar, it is now brighter than it was bi"f>re.
Oct. 31. 6h. 6'. Ten-feet reflector. Astar in the tail of the comet, which w© cull a, is much less bright than two others, h and c. without the tail.
Two oiher stars, d and e, towards tha south of/>and r, are in the following skirts of the tail, and are extremely faint.
7h. 20'. The star e Js now considerably blight, the tail having left it, while d, which is rather more involved than it was beibre, is hardly to be seen.
7h. 50'. The star a, toward which the comet moves, is involved in denser nebulosity than before, and is grown fainter.
d is involved in brighter nebulosty than before, but being near the margin, it will soon cmeFge.
8h. 35'. Being still more involved, the star a is now hardly visible.
e is quite clear of the tail, and is a considerable star; d remains involved.
9h. 10i. The star d is also emerged, hut ilte comet is now too low to estimate, the brightness of stars properly,
Nov. 2a. 7li. 36'. There is a star a within the light of the tail, near the bead of the comet, equal to a star b situate without the tail, but near enough to lie seen in the field of view with o. The path of the head of the comet leads towards ,;, and a more iutense brightness will coma upon it.
8h. 46'. The star o is now involved in the brightness near the head of the comet, and is no longer visible, except now audi then very faintly, by occasional imperfect
ilimpses; but the star b retains its former ngl't.
Nebulous Appearance of the Comet. Dec. 6. The head of the comet, viewto! with a mirror of twenty-four inches diameter, resembles now one of those ncbuIx, which in my catalogue* would hare been described, "a very large, brilliant, round nebula, suddenly much blighter in tic middle."
Dec. 10. Seven-feet reflector. The night being hue, and the'moon not risen, the comet resembles a very bright, large, irregular, round nebula, very gradually iii'ji !i brighter in the middle, with a faint bebulosicy on the south-preceding side.
Jan. 1, 1808. Seven-feet, "Very UrigAt, very large, verv gradually much brighter in the middle.1'
It I had not known this to be n comet, 1 should iiave added to my description of it as a nebula, that the centre of it might baotiM of very small stars; but ibis being impossible, I directed my ten-feet teleKwpc with a high power to (lie comet, in order to ascertain the cause of this appearance; in consequence of which I perceived several small stars shining through the nebulosity of the coma. Jan. 14. Seven feet. "Bright, pretty •regularround, brighter in the middle.*
Feb. 2. Ten-feet, twenty-four-inch ap» rture. "Wry bright, large, irregular rouiuj, verv gradually much brighter in tic middle." Tlieie is a faint diffused nebulosity mi the north preceding side; I take it to be the vanishing remains of the comet's tail.
Feb. 19. Couriderahlv bright; about
Jth of the fir-Id = 3' 30" " 111 diameter,
parluaUy brighter in the middle." Tim
Ml nebulosity in the plnce where the nil
used to be, still projects a little further
frum the centre than ill other directions.
Feb. 21. Less bright than on the 19th;
nearly nftl.esani'i size : gradually brighter
The nebulosity still a little
Hi the side where the tail used
• of the foregoing Ohersutkms.
1 the observations which are now
before us, »c may draw some inferences,
*bicb v onsidcrable importance
* .ill Heard l<-i the iuforuiatinn they give
- of the comet, hue
ature of its illumination.
d, and well defined disk,
1 tofit witheriJiiil briglit
wo material circiunstan
nr.clcus of this comet,
f ra ulauel, appeared in tkc
shape of a disk, which was experimentally found to be a real one, we have good reason to believe that it consuls of some condensed or solid body, the magnitude of which may be ascertained by calculation. For instance, we have seen, that its apparent diameter, the ]9th of October, till. 20', was not quite so large as that of the third satellite of Jupiter. In orderrhero fore to have some idea of the real magnitude of our comet, we may admit that its diameter at the time of observation was about 1", which certainly cannot lie far from truth. The diameter of the third satellite of Jupiter, however, is known to have a permanent disk, such as may at any convenient time be measured with all the accuracy that can be used ; and when the result of such a measure has given us the diameter of this satellite, it may by calculation be brought to the distance Iroin the earth at which, in my observation, it waa compared with the diameter of the comet, and thus more accuracy, if it should he re'iuiied, may be obtained. The following result of my calculation, however, appears to rue quite sufficient for the purpose of general information. From tlte perihcli.in distance 0-017-191, and the rest of the given elements of the comet, we find, that its distance from the ascending nolle on its orbit at the time of observation was 73° 45' 4-1'; and having also tire earth's distance from the same node, and the inclination of the comet's orbit, we compute by these data the angle at the sun. Then bv calculating in the next place the radius vector of the comet, and having likewise the distance of the earth troin the sun, we find by computation, tlmt the distance of tlic comet from the earth at the time of observation was 1169192, the mean distance of the earth being 1. Now since the disk of the comet was observed to subtend an angle of 1", which brought to the mean distance of the earth gives l'-109, and since we also know that the earth's diameter, which, according to Mr. Dolby, is 7913-2 miles*, subtends at the same distance an angle of 17"'2, we deduce from these prii*. cioles the real diameter of the comet, which is 538 miles.
Having thus investigated the magnitude of our comet, wc may in the next place also apply calculation to its illumination. The observations relating to the light or."
• Sec Philosophical Transactions for 1791, p. 2^)9. Mr Dalby gives the two semiuxei of the Earth, from a moan of which the abuvc diameter 791J loliv is hbuiucd.
the comet were made from the 4th of October to the 19th. In all which time the comet uniformly preserved the appearance of a planetary disk fully enlightened by the sun: it was every where equally bright, round, and well defined on its borders. Now as that part of the disk which was then visible to us could not possibly have a full illumination from the sun, t have calculated the phases of the comet tor the 4th and for the 19th : the result of ■which is, that on the 4rh the illumination was 119° 45' 9", and that on the 19th it had gradually increased to 121° 22* 40". Both phases appear to inc sufficiently defalcated, to prove that the comet did not shine by light reflected from the sun only; for, had this been the case, the deficiency, I think, would have been perceived, notwithstanding the sniullniss of the object. Those who are acquainted with my experiments on small silver globules will easilv admit, that the same telescope which could show the spherical form of balls, that subtended only a few tenths of a second in diameter, would surely not have represented a cometary disk as circular, if it had been as deficient as arc the figures which give the calculated appearances.
If these remarks are well founded we, are authorised to conclude, that the body of the comet on its surface is self luminous, from whatever cause this quality mav be derived. The vivacity of the light of the comet also had a much greater resemblance to the radiance of the stars, than to the mild reflection of the sun's beams from the moon, which is an additional support to our former inference.
The changes in the brightness of the small stars, when they are successively immerged in the tail or coma of the comet, or clear from them, prove evidently, that they are sufficiently dense to obstruct the free passage of star-light. Indeed if the tail or coma were composed of particles that reflect the light of the sun, to make them visible we ought rather to expect that the number of solid reflecting particles, required for this purpose, would entirely prevent our seeing any stars through them. But the brightness of the head, coma, and tail alone, will sufficiently account for the observed changes, if we admit that they shine not by reflection, hut by their owu radiance; for a faint object projected on a bright ground, or seen through it, will certainly appear somewhat fainter, although its ritys should meet with no obstruction in coming to the eye. Now, ns in this case we ure sure of the bright interposition of the parts of the co
met, but have no knowledge of floating particles, we ought certainly not to ascribe an effect to a hypothetical cause, when the existence of one, quite sufficient to explain the phenomena, is evident.
If we admit that the observed full illumination of the disk of the comet cannot be accounted for from reflection, we may draw the same conclusion, with respect to the brightness of the head, couia, and tail, from the following consideration. The observation of the 2d of February mentions, that not only the head and coma were still very bright, but that also the faint remains of the tail *ere visible; but the distance of the comet from the Earth, at the time of observation, was nearly 240 millions of miles*, which proves, I think, that no lijrut reflected from floating particles could possibly have reached the eye, without supposing the number, extent, and density of these panicles far greater than what can be admitted.
My lasi observation of the comet, on the 21st of February, gives additional support to what has been said; for at the time of this observation the comet was almost 2'9 times the mean distance of the sun from the earth f. It was also nearly 2-7 from the sun \. What chance then could rays going to the comet from the sun, at such a distance, have to be seen after reflection, by an eye placed at more than 27i> millions of miles § from the comet? And yet the instant the comet made its appearance in the telescope, it struck the eye ns a very conspicuous object.
'1 he immense tails also of some comet, that have been observed, and even that of the present one, the tail of which, on the 18th of October, was expanded over a space of more than nine millions of milca ||, may be accounted for more satisfactorily, by admitting them to consist of radiant matter, such as, for instance, the aurora borealis, than when we unnecessarily nscribe their light to a reflection of the sun's illumination thrown upon vapours supposed to arise from the body of the comet.
By the gradual increase of (he distauca of our comet, n-c have seen, that it assumed the resemblance of a nebula; and it is certain, that had I met with it in one
f The tun'« menn distance bring 1, tb»t of the comet was •J,B!l?o?,
J The coram'*, dijuuee friHn the sua waa i.,,6lUl96
of my sleeps of the gooes of the heavens, as it appeared on either of the days between the 6th of December mid jtfie 21st of February, it would have been put down in the list I have niven of nebula'. This remark cannot but raise a suspicion, that mine comets may hare actually been seen under a nebulous form, and as such have
been recorded in my catalogue?; and were it not a task of many years' labour, I should undertake a review of' all my nebula;, in order to see whether any of I hem were wantiuv:, or bad changed their place; which certainly would be an investigation that mie;ht lead to very interesting conclusions.
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