uniformly rejecting from all quarters, the air which surrounds it, in such a manner as to produce this circular and opaque covering. And perhaps this opaqueness may be caused by some particles which some current brings from without; perhaps also, the heat communicates to the moon the property of emitting by the pores in its surface, the particles by which this effect is produced.
“Comets arise either from the fact, that in the circumstances already stated, there are partial conflagrations in certain points of the heaven; or, that at certain periods, the heaven has above our heads a particular movement which causes them to appear. It may also be the case, that being themselves endowed with a peculiar movement, they advance at the end of certain periods of time, and in consequence of particular circumstances, towards the places which we inhabit. The opposite reasons explain their disappearance.
“Certain stars return to the same point in accomplishing their revolutions; and this arises, not only as has been sometimes believed, from the fact of the pole of the world, around which they move, being immoveable, but also from the fact that the gyrations of the air which surrounds them, hinder them from deviations like the wandering stars. Perhaps also, this may be caused by the fact, that except in the route in which they move, and in which we perceive them, they do not find any material suitable to their nature. One may also explain this phænomenon in many other manners, reasoning according to sensible facts; thus, it is possible that certain stars may be wandering because that is the nature of their movements, and, for the same reason, others may be immoveable. It is also possible, that the same necessity which has originally given them their circular movement, may have compelled some to follow their orbit regularly, and have subjected others to an irregular progress; we may also suppose that the uniform character of the centre which certain stars traverse favour their regular march, and their return to a certain; and that in