meter of the moon; the comet that was seen in January, 1831, will furnish an illustration of their diminution; on the 7th day it shone with the brilliancy of a star of the second magnitude; on the 26th it exhibited a pale nebulous appearance, and could only be seen with the telescope. It must, however, be here observed, that this increase and diminution arise not merely from the advance or retreat of the comet, in relation of distance, but from this and the solar action, which raises the nebulous matter, and in some way influences the nucleus as the comet approaches, which matter is again condensed as the comet retreats from the sun; distance diminishes, and proximity increases the angle under which a body is seen; so that in comets there are two principles combining to affect their apparent and real magnitudes.

Planets move nearly in the same plane with one another, their orbits not being much inclined to the earth's path or ecliptic; the larger planets, Jupiter and Saturn, being each respectively 1° 19′ and 2o 30'; whereas, comets make all possible angles with the ecliptic, from below 10° to paths at right angles to the courses of the planets: the orbit of the comet of Encke is inclined 13° 20′, that of Biela, 13° 32′, that of Halley, 17° 39', the comet of 1680, 61° 22′, and one that was observed in 1818, had its orbit inclined to the ecliptic in an angle of 89° 47'.

Planets move all the same way in the order of the signs, that is from west to east; comets move both ways, direct and retrograde; nor are these contrary motions merely apparent, being combined with the earth's motions, which make even the planets appear to move

backwards, which movement they have ad; but their motion, in their orbits, are absolutely in des

which the planets move, and contrary

Jirect comet will appear to move
of the signs when it is passing
sun, and its absolute motion exceeding
this was the case with the comet of
passed from Aries towards Pisces, Aquari
comus; when it arrived on the opposite side
had it then have been visible, it would a
in its direct course.

To the planets the disc of the sun appears nearly of the same magnitude throughout the whole of their repective annual courses; whereas, some comets, when at their aphelion, would have the sun not much larger than a star of the first magnitude, and when returning and arrived at the orbit of Uranus, it would see the san about the size that Venus is when largest; at the orbit of Saturn, the solar angle would be 3'; Jupiter, 6; Mars, 21; Earth, 32; Venus, 46; Mercury, 80'; and to the comet of 1680, when in perihelion, the sun would appear 100° in breadth, or 40,000 times as largo as the Sun appears to us.

The planets move with near the throughout the whole of their respect

is very far from being the case w
1680, when in perihelion, or that po
moved 1,768,000 feet in a second of t
aphelion, or most remote from the sun,
the same time. The great comet of 193

is eleven months describing a second of space.

motion little exceeding the swiftness walking