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gained, because the exertion of a power depending on the quantity of light was obstructed, which I found was here of greater cousequence than the increase of maguitude.

Illumination of the Nucleus." Oct. 4, 6h. 15'. The nucleus is apparently round, and equally bright all over its disk. I attended particularly to its

roundness.

Oct 18. The nucleus is not only round, but also every where of equal brightness. Oct. 19. I see the nucleus again, perfectly round, well defined, and equally luminous. Its brilliant colour in my tenfeet telescope is a little tinged with red; but less so than that of Arcturus to the naked eye.

Magnitude of the Nucleus.

Oct. 26. In order to see the nucleus as small as it really is, we should look at it a long while, that the eye may gradually lose the impression of the bright coma winch surrounds it. This impression will diminish gradually; and when the eye has got the better of it, the nucleus will then be seen most distinctly, and of a determined magnitude.

Oct. 4. With a seven-feet reflector I estimated the diameter of the nucleus of the comet at first to be about five seconds; but soon after I called it four, and by looking at it longer, I supposed it could not exceed three seconds.

Oct. 6. Ten-feet reflector, power 221. The apparent disk of the comet is much less than that of the Georgian planet, which being an object I have seen so often with the same instrument, and magnifying power, this estimation from memory cannot be very erroneous.

Oct. 5. Micrometers for measuring very small diameters, when high magnifying powers cannot be used, being very little to be depended upon, I erected a set of sealing-wax globules upon a post at 2422 inches from the object mirror of my tenfeet reflector, and viewed them with an eye glass, which gives the instrument a power of 221, this being the same which I had found last night to show the nucleus of the comet well. I kept them in their place all the day, and reviewed them from time to time, that their magnitudes might be more precisely remembered in the evening, when I intended to compare the appearance of the nucleus with them.

On examining the comet, I found the diameter of its nucleus to be certainly less thu the largest of my globules, which, being '0466 of an inch, subtended an an gle of 397 at the distance of the tele scope in the day time,

Comparing the nucleus also with the impressions which the view of the second and third had left in my memory, and of which the real diameters were '0325 and 0290 of an inch, and magnitudes at the station of the mirror 2.77 and 2·47, I found, that the comet was almost as large as the second, and a little larger than the third.

Oct. 18. The nucleus is less than the globule which subtends 2-77.

Oct 19. The air being uncommonly clear, I saw the comet forty minutes after five; and being now at a considerable altitude, I examined it with 289, and having but very lately reviewed my globules, judged its diameter to be not only less than my second globule, but also less than the third: that is, less than 2:47.

Oct. 6. The twenty-feet reflector, notwithstanding its great light, does not show the nucleus of the comet larger than the ten-feet, with an equal magnifier, makes it.

Oct. 28. My large ten-feet telescope, with the mirror of twenty-four inches in diameter, does not increase the size of the nucleus.

Oct. 6. Being fully aware of the objections that may be made against the method of comparing the magnitude of the nucleus of the comet with objects that cannot be seen together, I had recourse to the satellites of Jupiter for a more decisive result, and with my seven-feet telescope, power 202, I viewed the disk of the third satellite and of the nucleus of the comet alternately. They were both already too low to be seen very distinctly; the diameter of the nucleus however appeared to be less than twice that of the satellite.

Oct. 18. With the ten-feet reflector, and the power 221, a similar estimation was made; but the light of the moon would not permit a fair comparison.

Oct. 19. I had prepared a new tenfeet mirror, the delicate polish of my former one having suffered a little from being exposed to damp air in nocturnal ob servations. This new one being uncom monly distinct, and the air also remarkably clear, I turned the telescope from the comet to Jupiter's third satellite, and saw its diameter very distinctly larger than the nucleus of the comet. I turned the telescope again to the comet, and as soon as I saw it distinctly round and well defined, I was assured that its diameter was less than that of the satellite.

6h. 20. I repeated these alternate observations, and always found the same result. The night is beautifully clear, and

the

the moon has not yet risen to interfere with the light of the comet.

Nov. 20. With a seven-feet reflector and power only 75, I can also see the nucleus; it is extremely small, being little more than a mere point.

Of the Head of the Comet. When the comet is viewed with an inferior telescope, or if the magnifying power, with a pretty good one, is either 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 the head of the comet 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 magnifying power.

With a good telescope, I saw in the centre of the head a very small well-defined round point.

Nov. 20. The head of the comet is now less brilliant than it has been.

Of the Coma of the Comet. The coma is the nebulous appearance Surrounding the head.

Oct. 19. By the field of view of my reflector, 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 diametery is now about 4.45.

Of the Tail of the Comet. Oct. 18. 7h. With a night glass, which has a field of view of nearly 50, 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. 20. The tail of the cornet is still of a considerable length, certainly not less than 2 degrees.

Oct. 26. The tail of the comet is considerably longer on the south-preceding, than on the north-following side.

It is not bifid, as I have seen the comet of 1769 delineated by a gentleman who had carefully observed it*.

Oct. 28. Seven-feet reflector. The south-preceding side of the tail in all its length, except towards the end, is very well defined; but the north-following side is every where hazy and irregular, especi ally towards the end; it is also shorter than the south-preceding one.

The shape of the unequal length of the sides of the tail, when nttentively viewed, is visible in a night glass, and even to the naked eye.

• Dr. Lind of Windsor,

Oct. 31. 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 23′ of a degree. Of the Density of the Coma und Tail of

the Comet.

Many authors have said, that the tails of comets are of so rare a texture, as not to affect 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, I 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 inches aperture. A small star within the coma is equally faint with two other stars that are on the north following side of the comet, but without

the corna.

7h. 30. The coma being partly removed from the star, it is now brighter than it was before.

Oct. 31. 6h. 5. Ten-feet reflector. A star in the tail of the comet, which we call a, is much less bright than two others, b and c, without the tail.

Two other stars, d and e, towards the south of b and c, are in the following skirts of the tail, and are extremely faint.

7h. 20. The star e is now considerably bright, the tail having left it, while d, which is rather mare involved than it was before, is hardly to be seen.

7h. 50'. The star a, toward which the comet moves, is involved in denser nebu→ losity than before, and is grown fainter.

dis involved in brighter nebulosty than before, but being near the margin, it will

soon emerge.

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. 10. The star d is also emerged, but the comet is now too low to estimate the brightness of stars properly.

Nov. 22. h. 33. There is a star a within the light of the tail, near the head of the comet, equal to a star b situate without the tail, but near enough to be seen in the field of view with a. The path of the head of the comet leads towards a, and a more intense brightness will come upon it.

8h. 46'. The star a is now involved in the brightness tear the head of the comet, and is no longer visible, except now and then very faintly, by occasional imperfect

glimpses;

glimpses; but the star & retains its former light.

Nebulous Appearance of the Comet. Dec. 6. The head of the comet, view ed with a mirror of twenty-four inches diameter, resembles now one of those nebule, which in my catalogues would have been described, "a very large, brilliant, round nebula, suddenly much brighter in the middle."

Dec. 18. Seven-feet reflector. 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 middle, 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 it as a nebula, that the centre of it might consist of very small stars; but this being impossible, I directed my ten-feet telescope with a high power to the comet, in order to ascertain the cause of this appearance; in consequence of which I per ceived several small stars shining through the nebulosity of the coma.

Jan. 14. Seven feet. "Bright, pretty large, irregular round, brighter in the mid

dle."

Feb. 2. Ten-feet, twenty-four-inch aperture. "Very bright, large, irregular round, very gradually much brighter in the middle." There is a faint diffused nebulosity on the north preceding side; I take it to be the vanishing remains of the

comet's tail.

46

Feb. 19. Considerably bright; about th of the field=9' 26" in diameter, gradually brighter in the middle." The faut nebulosity in the place where the tail 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 brighter it the middle. The nebulosity still a little projecting on the side where the tail used to be.

Result of the foregoing Observations.

From the observations which are now before us, we may draw some inferences, which will be of considerable importance with regard to the information they give us, not only of the size of the comet, but also of the nature of its illumination,

A visible, round, and well defined disk, s'ining in every part of it with equal bright ness, elucidates two material circumstances; for since the nucleus of this comet, like the body of a planet, appeared in the

shape of a disk, which was experimentally found to be a real one, we have good reason to believe that it consists 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 19th of October, 6h. 20', was not quite so large as that of the third satellite of Jupiter. In order therefore 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 be 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 calcu lation be brought to the distance from the earth at which, in my observation, it was compared with the diameter of the comet, and thus more accuracy, if it should be required, may be obtained. The following result of my calculation, however, appears to me quite sufficient for the purpose of general information. From the perihelion distance 0-647-491, and the rest of the given elements of the comet, we find, that its distance from the ascending

node on its orbit at the time of observation was 73° 45′ 44'; and having also the 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 by calculating in the next place the radius vector of the comet, and having likewise the distance of the earth trom the sun, we find by computation, that the distance of the 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 1'-169, and since we also know that the earth's diameter, which, according to Mr. Dalby, is 7913-2 miles, subtends at the same distance an angle of 17"-2, we deduce from these prin ciples the real diameter of the comet, which is 538 miles.

Having thus investigated the magnitude of our comet, we may in the next place also apply calculation to its illumination. The observations relating to the light of

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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, I have calculated the phases of the comet for the 4th and for the 19th: the result of which is, that on the 4th the illumination was 119° 45' 9", and that on the 19th it had gradually increased to 121° 22′ 40′′. Both phases appear to me 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 smallness of the object. Those who are acquainted with my experiments on small silver globules will easily 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 are 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 may 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 parti cles, 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, but by their own radiance; for a faint object projected on a bright ground, or seen through it, will certainly appear somewhat fainter, although its rays should meet with no obstruction in coming to the eye. Now, as in this case we are 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 illu mination 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, coma, 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 were 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 light reflected from floating particles could possibly have reached the eye, without supposing the number, extent, and density of these particles far greater than what can be admitted.

My last 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 +. 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 275 millions of miles from the comet? And yet the instant the tomet made its appearance in the telescope, it struck the eye as a very conspicuous object.

The immense tails also of some comets 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 miles, may be accounted for more satis factorily, by admitting them to consist of radiant matter, such as, for instance, the aurora borealis, than when we unnecessarily ascribe 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 the distance of our comet, we have seen, that it assu med the resemblance of a nebula; and it is certain, that had I met with it in one

239894939.

The sun's mean distatice being 1, that of the comet was 2.89797,

The comet's distance from the sun was 2.683196.

975077889.

9160542.

of my sweeps of the zones of the heavens, as it appeared on either of the days between the 6th of December and the 21st of February, it would have been put down in the list I have given of nebulæ. This remark cannot but raise a suspicion, that some comets may have actually been seen under a nebulous form, and as such have

been recorded in my catalogues; and were it not a task of many years' labour, I should undertake a review of all my nebulæ, in order to see whether any of them were wanting, or had changed their place; which certainly would be an investigation that might lead to very interesting conclusions.

LIST OF NEW PUBLICATIONS IN FEBRUARY.

As the List of New Publications, contained in the Monthly Magazine, is the ONLY COMPLETE LIST PUBLISHED, and consequently the only one thut can be useful to the Public for Purposes of general Reference, it is requested that Authors and Publishers will continue to communicate Notices of their Works (Post paid), and they will always be faithfully inserted, FREE of EXPENCE.

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A History of France, from the Commencemest of the Reign of Clovis to the Peace of Campo Formio 1797, after the Manner of the History of England, in a Series of Letters from a Nobleman to his Son. 12mo. 58. 6d.

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A Letter to John Haygarth, M.D. from Colin Chisholm, M.D. Author of an Essay on the Pestilential Fever, exhibiting further Evidence of the infectious Nature of this fatal Distemper in Grenada during 1793, 4, 5, and 6, and in the United States of America, from 1793 to 1805; in order to correct the pernia cious doctrine promulgated by Dr. Edward Miller and other American Physicians, relative to this destructive Pestilence. 8vo. 6s.

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