NOTE.

The distances of the moon have been corrected from the effects of refraction, parallax and the spheroidal figure of the earth. In calculating the refractions, allowances have been made for the state of the barometer and thermometer.—To deduce the longitude of the moon, use has been made of the longitudes of the stars lately determined by Maskelyne.

No. LIV.

Observations on the Comet of 1807-8. By William Dunbar.

Read November 18th, 1808.

THESE observations were made in latitude 31° 27' 48" N. and 6 5′ 50′′ nearly, west of Greenwich. The instrument principally used for taking distances was a circle of reflection by Troughton of London, graduated by the Vernier to ten seconds of a degree, and firmly supported upon a pedestal, adapted to every necessary movement; the observations were made with the most scrupulous care, and as the pedestal afforded every desirable facility, no observation was written down until it had been re-examined several times by the separation and reunion of the images. The clock was regulated to mean time.

This comet was first seen here about the 20th of September 1807, and Seth Pease Esq. Surveyor of the Mississippi Territory, began to make observations on it the 22d of the same month; and as I have the greatest reliance on the correctness of this gentleman (who is an excellent astronomer) I shall here give his observations which precede my own.

This evening I observed an emersion of the 1st satellite of Jupiter, with a six feet Gregorian reflector, power 128, at 8 27′ 58′′ mean time; a fine observation; the sky was very serene: from the darting of the first ray from the satellite, it seemed a mere point of light for 15 seconds of time; perhaps with telescopes in general use the satellite might have remained invisible during those 15 seconds, which would affect the longitude resulting from the observation; from the above, the longitude deduced is 6 5′ 55′′ west of Greenwich.

The following are my own observations:

1807, October 2d. The reflecting telescope, power 128, being directed to the comet, shewed the nucleus and coma with tolerable distinctness; the idea produced in the mind of the observer, was that of a round body in combustion, which had produced so much smoke as to obscure the nucleus; the smoke seemed to be emitted in every direction; but, as if it met on one side with a gentle current of air, the smoke seemed to be repelled and bent round the nucleus, escaping on the opposite side, in the direction of the tail. To the naked eye, the comet could not be seen earlier in the evening than a star of the second or third magnitude, although its disk was considerably larger; with the telescope, the nucleus did not seem to be much more than one third, certainly not one half of the planet Mars, which had been lately observed. The Coma seemed to be at least ten times the magnitude of the nucleus; an imperfect measure was taken of the tail, which was about 63′ in length. The coma appeared to be, in a certain degree, illuminated from the nucleus, and the whole was compared by many persons to a distant building on fire. This evening, the following observations were made:

4th. This evening there was an occultation of the planet Mars by the moon, at 72′ 1′′ mean time: the moon was low, involved in the grosser horizontal atmosphere, and, apparently, almost touching the tops of the forest trees.

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The comet was dim this evening, caused by a mist or smoke which obscured the lower parts of the atmosphere, and became invisible before any more observations could be taken.

The atmosphere was again so misty, or rather smoky, that no more observations could be taken: the cause of the smoky state of the atmosphere at this season, in our country, is the setting fire to the dry grass of the immense prairies or savannahs, and pine wood forests, in our neighbourhood; the smell of the burning pine, and other resinous aromatick vegetable matters is sometimes very strong, although the conflagrations which occasion it, are not supposed to be nearer than from 50 to 100 miles: dense clouds are frequently formed of the smoky vapour, from whence proceed violent tempests, with thunder and lightning, and torrents of rain of a brownish black colour.

The apparent length of the comet's tail was this evening about 2° 43' 30".

The splendor and apparent magnitude of the comet visibly diminish; the nucleus seems reduced to little more than half its first observed magnitude.

December 6th. Indisposition prevented observation for some time past. This evening being fine, I directed the reflecting telescope to the comet; the nucleus is now much diminished in apparent magnitude; I compared it with a star of the sixth magnitude in the Swan, which was within the field of view at the same time, their apparent diameters were nearly equal, but the comet is become so dim, as to be seen by the naked eye only in a pure atmosphere, with favourable circumstances: the weather being cold and damp, my state of health did not permit taking any distances: the coma is yet considerable, but the tail is no longer visible, one would be inclined to say, as the comet recedes from the sun, that the tail is called in (as it were) to add to the magnitude of the coma; for certainly the latter is but very little diminished in proportion to the nucleus.

In order to supply my own deficiencies, I shall here introduce the observations of Mr. Pease (on whose correctness I place the greatest reliance) during the time my own were interrupted by ill health.

The comet now became too obscure to make any observations upon it, with the sextant, although Mr Pease has given two of the evening of the 22d of January, which he says are true only to five or six minutes, as follows,

The observations which follow are extracted from my own journal.

January 5th. The comet is no longer visible to the naked eye; though having pointed the telescope to its calculated place, I discovered it a little to the S. E. of 2 Cygni; but this star was not near enough in declination, to take the comparative position of the comet, with the micrometer of the reflecting telescope, in the manner pointed out by Dr. Maskelyne: however, as an approximation is often desirable, I directed to the comet and star, a very good small achromatic telescope, magnifying eleven times, and found that the two objects were distant from each other about two thirds of the diameter of the field of view of the telescope, and having placed the comet and star across the center of the field, and opening the left eye, I found that a line joining the star and comet, produced, would pass through Pegasi; the angle of the field of view of the telescope having been ascertained to be 2° 18′ 26", two thirds of which are 1° 32′ 18′′ the distance of the comet from 2 Cygni in the direction Pegasi; from whence a good approximation of the place of the comet may be deduced. Note, the star - 1 Cygni is marked in Wollaston's catalogue, of the fourth magnitude and 2 Cygni of the fifth magnitude, but 2 is now the larger; the stars ought, therefore, to change designations.

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The nucleus of the comet is yet to be distinguished by the reflecting telescope, but as small as a star of the seventh mag