Glasgow Observatory, Oct. 16. SIR-In the Glasgow Courier of October 5, I had the honour of submitting to the public the results of the joint labours of Mr. Cross and myself, for the preceding month, on the comet, at the Glasgow Observatory. In The Star newspaper of October 11, ap. peared for the first time the elements of the orbit, as determined by the celebrated Burckhardt, Member of the National Institute.

It is a duty which I owe to the skill and the unwearied exertions of my associate Mr. Cross, to this patriotic establishment, and also to this country, hitherto considered by the French mathematicians and astronomers unequal to the primary solution of this difficult problem, to state the following facts:-On October 8, at eight hours fifteen minutes, by observations made here, with every precaution to insure the utmost accuracy, the comet had deviated 42 degrees 18 minutes from the longitude which Burckhardt's elements assign for that instant. On October 14, at two o'clock in the morning, the longitude, as deduced from a most satisfactory transit, was 206 degrees 42 minutes. By the French computation it ought to have been 248 degrees 1 minute, differing from nature by 41 degrees 19 minutes. By our elements, which have received a partial correction from my observations since the 5th, the coincidence on the 8th, at the same time, was within 15 minutes, and on the 14th, within 13 minutes. Our computed latitudes on the 13th agree to a minute with observation, while those of Burckhardt differ by 3 de grees or 180 times that quantity.

The examination of both has been made by the excellent tables of the parabola, constructed by Delambre, imperial observer at Paris. It is in the longitude of the perihelion that the chief discordance exists between the French elements and ours, and this amounts to about 31 degrees; the former being, in our judgment, too small by this quantity.

The comet has been continually approaching the earth for many weeks. From September 15th till October 14th, its decrease of distance amounted to 25 millions of miles, yet its brilliancy and the magnitude of its tail have gone on diminishing, as Burckhardt properly remarked. Persons ignorant of astronomy would naturally infer from this diminution the recedure of the comet from us, as, from its increase they conjectured its approach. Astronomers laugh at such idle dreams when applied to a demonstrative science, in

which conjecture has found no place since the days of Newton. Its first principles teach, that these phænomena arise from the comet's varying distance from the sun. At the period of the perihelion passage these bodies are known uniformly to attain their maximum of size and brightness.

[Tilloch's Phil. Mag.]

ANDREW URE.

CHAP. XIII

OF SOLAR AGENCY IN THE PRODUCTION OF COMETIC PHÆNOMENA.

As we we are now in a great measure acquainted with the physical construction of the different parts of the present comet, and have seen many successive alterations that have happened in their ar rangement, it may possibly be within our reach to assign the probable manner in which the action of such agents as we are acquainted with has produced the phænomena we have observed.

In its approach to a perihelion, a comet becomess exposed to the action of the solar rays, which, we know are capable of producing light, heat, and chemical effects. That their influence on the present comet has caused an expansion, and decomposition of the cometic matter, we have experienced in the growing condition of the tail and shining quality of its light, which seems to be of a phos phoric nature. The way by which these effects have been produced may be supposed to be as follows.

The matter contained in the head of the comet would be dilated by the action of the sun, but chiefly in that hemisphere of it which is immediately exposed to the solar influence; and being more increased in this direction than on the opposite side, it would become eccentric, when referred to the situation of the body of the comet; but as the head is what draws our greatest attention, on account of

its brightness, the little planetary body would appear to be in the eccentric situation in which we have seen it.

. Now, as from observed phænomena, we have good reason to believe the comet to be surrounded by a very extensive, transparent, elastic atmosphere; the nebulous matter, which probably, when the couiet is at a distance from the perihelion, is gathered about the head in a spherical form, would on its approach to the sun be greatly rarefied, and rise in the cometic atmosphere till it came to a certain level, where it could remain suspended, for some time, exposed to the continued action of the sun.

In this situation we have had an opportunity of seeing the transparent atmosphere, which, but for the suspension of the nebulous matter, we might never have discovered and indeed, how far it may extend beyond the region which contained the shining substance, we can have no observation to ascertain, on account of its transparency. In consequence of the darkish interval, occasioned by the atmospheric space, the suspended light appeared to us in the shape of a very bright envelope.

The brilliancy of the envelope, and its yellowish colour, so different from that of the head, and probably acquired by its mixture with the atmospheric fluid, are proofs of the continued action of the sun upon the luminous matter, already in so high a state of rarefaction; and if we suppose the attenuation and decomposition of this matter to be carried on till its particles are sufficiently minute, to receive a slow motion from the impulse of the solar beams, then will they gradually recede from the hemisphere exposed to the sun, and ascend in a very moderately diverging direction towards the regions of the fixed stars.

That some such operation must have been carried on, is pretty evident from our having seen the gradual rise, and increased expansion of the tail of the comet; and if we saw the shining matter, while suspended in the cometic atmosphere, in the shape of an envelope, it follows that, in its rising condition, it would assume the appearance of those two luminous branches which we have so long observed to inclose the tail of the comet.

The seemingly circular form, and the stream-like appearance of the luminous matter, having been already explained, we may now see the reason why it can rise in no other form than the conical; for

a whole hemisphere of it being exposed to the action of the sun, it must of course ascend equally every where all around it.

That the luminous matter ascending in the hollow cone, received no addition to its quantity from any other source than the exposed hemisphere, we may conclude from its appearance; which notwithstanding the great circumference of the cone it filled, at the altitude of 6 degrees from the head, was never seen with increased lustre; although the diameter of an annular section of it, in that place, must have been nearly 15 millions of miles, and was but little more than half a million at its rising from the envelope.

This consideration points out the extreme degree of rarefaction of the luminous matter about the end of the tail; for its expansion while still much confined in the streams, at the altitude which has been mentioned, must have exceeded the deusity it had at rising about 524 times; but when afterwards it extended itself so as to produce nearly an evenly scattered light over the whole compass of the end of the tail, we may easily conceive to what an extreme degree of rareness its expansion must have been carried.

The vacancy occasioned by the escape of the nebulous matter, which after rarefaction passed from the hemisphere exposed to the sun into the regions of the tail, was probably filled up, either by a succession of it from the opposite hemisphere, or by a rotation of the comet about an axis; and the gradual decomposition of this matter would therefore be carried on as long as any remained to replace the deficiency.

That such a kind of process took place, seems to be supported by the observations which were made during the regression of the comet from its perihelion. For the space between the branches of the tail, very near the head of the comet, became gradually of a darker appearance than before; which indicated the absence of the nebu lous matter that had formerly been lodged there.

A rotatory motion of the comet, which has been suggested, would also explain the frequent variations in the length of the opposite branches which inclosed the tail; for if any portion of the cometary matter should be more susceptible of being thrown into a luminous decomposition than some others, a rotatory motion would bring such more susceptible matter into different situations, and cause a more or less copious emission of it in different places.

The additional short and faint double streams of nebulous light which issued from the vertex or side of the enfeebled envelope, in the gradual regress of the comet, tend likewise to add probability to the conception of a rotatory motion; for the changeable appearance of the situation of these streamlets might arise from a periodical exposition of some remaining small portions of less rarefied matter, when nearly the whole of it had been exhausted.

[Herschel, Phil. Traus. 1812.]

CHAP. XIV.

ON THE RESULT OF A COMET'S PERIHELION* PASSAGE.

AFTER having given a detail of phænomena, and entered into a research of the most likely manner in which they were produced, I shall only mention what appears to me to be the most probable consequence of the perihelion passage of a comet.

The quality of giving out light, although it may always reside in a comet, as it does in the immensity of the nebulous matter, which I have shown to exist in the heavens, is exceedingly increased by its approach to the sun. Of this we should not be sensible, if it were not accompanied with an almost inconceivable expansion and rarefaction of the luminous substance of the comet about the time of its perihelion passage.

It is admitted, on all hands, that the act of shining denotes a decomposition in which at least light is given out; but that many other elastic volatile substances may escape at the same time, especially in so high a degree of rarefaction, is far from improbable.

Then, since light certainly, and very likely other subtile fluids,

* The passage in which it approaches nearest to the sun: in which sense the term stands opposed to aphelion, constituting the comet's highest or most dis. tant point from the sun.-Editor.