length on the 6th of December, and on the 2d of February the tail was in a vanishing state, and it was observed on the 26th and 28th of October, that this was longest on the south preceding side, where it was well defined, except towards the extremity.

At the time of the observation on the 19th of October, its distance from the ascending node was 73° 45' 44", at which the distance from the earth was 1,169,192 or 111 millions of miles, and about a quarter when it subtended an angle of one second; and if the Earth's diameter (according to Mr. Dalby, see Philos. Trans. for 1791, p. 239) be 79,132 miles, and at this distance must subtend an angle of 17′′2, from these data the diameter of the Comet will be found to be no more than 538 miles, which is much less than has generally been imagined. But on the 2d of February, 1808, its distance from the earth was not less than 240 millions of miles. On Dr. Herschel's first sight of the Comet, on the 4th October, 1807, it had the appearance of a gibbous moon, the enlightened part of which was observed to be 119° 45′ 9′′, or nearly two-thirds of the diameter, which had extended on the 19th to 124° 22′ 40′, being more than 4 degrees, exceeding two thirds: but he nevertheless concludes, from its continual brilliance, similar to the fixed stars, when it was more remote, that it certainly possessed some self-luminous properties, besides what it could possibly have by reflection from the Sun. He calculated also that on the 9th of October the tail expanded over a space, considering its distance from the earth at that time, equal to 9,160,542 miles.

As the Moon is the fountain of radical moisture and conveys the influence of the heavenly bodies, we presume that no ,small pleasure will be afforded to the admirers of Astronomy by the insertion of the following observations on her by a gentleman, whose general knowledge and particular devotion,

to this divine science, render him equally an ornament to the circle in which he moves, and an honor to his country.-ed.

ON THE MOON.

Ir is the property of the Moon to accompany the Earth in its orbit round the Sun, notwithstanding the orbit in which he revolves round our globe. The diameter, by the most accurate calculation possible, is 2175 miles, the circumference 6831 miles, and the mean distance from the Earth about 240,000 miles. The time in which she moves round her axis is about 27 days, 7 hours, and 43 minutes; but in this time the Earth will have moved forward about 27 degrees in her orbit. Hence, it will be about 29 days, 12 hours, and 44 minutes, before the Moon will obtain the same relative situation with respect to the Earth and Sun.

This is the mean time between the period of one new Moon and another; though it will sometimes occur a little sooner, when the Moon's motion is quickest, in that interval after she has completed her revolution round her axis to the time of her next conjunction, but somewhat later, when she is then farthest from the Earth, and on that account slower in her motion.

Another known property of the Moon is, that she appears to us, at different points of her revolution, of a very different shape. After her change or conjunction, or what we call the new Moon, she must proceed to a certain distance in her orbit, so as to be out of the Sun's beams, before she can be, in any respect or degree, visible to us on the Earth. The quicker she is in motion, the sooner will this distance be accomplished. When the time of the change happens at a very early hour of the night, or, at least, soon after midnight, with a favourable velocity of motion, and a transparent air, the new Moon has been sometimes discovered towards the west, after Sun-setting, on the second

evening; but otherwise, it will seldom be seen until the third evening, especially when the change occurs in the latter part of the night, or in the day. When it is first seen in this situation, it will appear very narrow and horned, and only that part will be seen which is nearest to the Sun; but will every evening become broader and broader, and be seen before its setting longer and longer, till one half of the face appears towards us, when she will have attained to her first quarter. From that time she will daily become more and more gibbous, until she appears with a full face, when she will shine through the night, and then will be the time of full Moon. In like manner, as the Moon advances, and gets nearer to the Sun the other way, she will daily decrease until she becomes invisible for about two days, a little more or less, before the next change or conjunction.

In the revolutions which the Moon makes successively in her orbit, it is also obvious that she does not move in the same direction with the Earth. In this case, the Sun would be eclipsed by her every new Moon, and the inhabitants of our Earth would be deprived of his light by such an interposition. From the same cause it happens, that the Moon is not eclipsed by the intervention of the Earth at every full Moon, when that body is between the Sun and Moon. It has therefore been found by continued observation, that the two directions cross or intersect each other in points exactly opposite, and in an angle not much exceeding five degrees. But when these conjunctions or oppositions happen in those parts of the orbits so near to these points of intersection, as to become less than the sum of half the diameters of the Sun and Moon; though the directions from the angle just mentioned, if their distance in any other part of the Earth is less than about 32 minutes, an eclipse may be reasonably expected, either of the Sun at the new Moon, or of the Moon at the full Moon. At no other time can they possibly happen. But these things are only stated generally at present, of which

NO. III.

a more particular and exact notice and illustration shall be given hereafter.

These points, near which the Earth and Moon must be, in their respective orbits, to occasion eclipses, are called the nodes, or knots, as it were, to connect the two orbits together in two opposite paths; and if these nodes or points continued the same, the eclipses would happen in the same manner, or at the same seasons of the year successively. But it is found by observation, that these nodes also have a retrograde motion, and do not remain fixed to any particular point of the ecliptic, or in any special degree and minute of any of the twelve signs. This retrograde motion is such as to complete one revolution, or to pass through every sign and degree of the ecliptic in 18 years, 224 days, and about 3 hours. A daily motion of about 3 minutes and 11 seconds backward, will occasion the succession of the same eclipses to recede backward also, in the order of the signs, and consequently in the order of the months; though with some slight variation as to their being the same identical eclipses, of which there will be occasion to take notice hereafter, when the eclipses come under our immediate consideration.

It has been observed, that the surface of the Moon presents to the inhabitants a great variety of hiils and mountains. It is also manifest, that there are some small and many large spots of a brighter hue, interspersed in the darker portions of the face; which appear to stand, or push off at several distances from the boundary line. In the lucid part also are many small spots of a sombre cast, dispersed in various parts; and from their appearance to us, it cannot be doubted that some of them must be deep and hollow cavities, because their dark sides are always the nearest to the Sun, and their bright sides farther removed from him. The contrary must have appeared had they been mountains; for then the brighter sides would be seen nearest to the Sun, and the darker sides would be most remote.

Some of our writers on this subject have presented to the public a delineation or figure of the face of the Moon which is visible to the Earth. In a calm and clear night, when the Moon is at the full, or on the opposite side of the Earth from the Sun, it cannot be difficult to mark out the appearances of oceans, seas, and lakes; to parcel out the parts which rationally enough may be considered as countries or islands, and to distinguish the most remarkable mountains and hills which appear on the Moon's enlightened face. I have such a delineation before me at the present moment, in which are distinguished six such mountains, and denominated according to the most noted eminences on the surface of our globe; as Mount Sinai, Mount Taurus, Mount Siphor, Mount Etna, Mount Apenninus, and Mount Olympus, some of which are undoubtedly higher than any on our earth. The shadow of some of them may be very distinctly seen by the aid of a good telescope, as it appears longer or shorter on the face of the Moon's surface, and as the Moon is either approaching to or receding from the Sun. And it is recorded, that Ricciolous, upon viewing the Moon about four days old, particularly observed the point of a hill, which he called St. Catherine, near the north part of Mount Taurus, to be distinctly illuminated, being at that time distant from the surface apparently about one eighth part of the Moon's semidiameter; and this by the rules of trigonometry, which will be explained in another part of our plan, will give no less than three miles for the height of that mountain, which is much higher than any known mountain on our globe.

The various inequalities or interruptions of a right line on the surface of the Moon, at the separation of the light from the dark parts, appearing to us to be irregularly toothed or jagged, can only be accounted for by concluding, that the Moon's surface is very uneven, and much broken, and diversified into hills and cavities, or hollows. It is not in one part or one line of this separation only that these inequalities