We see, during a month before and after the equinox of March, when the sun is set and the twilight disappeared, a sort of whitish light resembling the tail of a comet. It is seen before the dawn and sun-rise, towards the equinox of September, and morning and night towards the winter solstice. At other times, as I have before said, the twilight conceals it; for we have reason to believe it always exists. It has lately been conjectured that it is produced by a large mass of matter, somewhat dense, which environs the sun for a certain extent. The greatest part of his rays penetrate this covering, and come to us in a straight line; but some of these rays by striking against the internal surface are reflected back to us, either before the direct rays can reach us in the morning, or after they have ceased to enlighten us in the evening. As these reflected rays come from a higher region than the direct ones, it is therefore earlier when we receive them, and later before we lose them. On this ground I must retract what I said on the probability of the moon having no twilight, for want of a surrounding atmosphere as dense as that of the earth. She is no loser by it, if she can receive a twilight through this thick air which surrounds the sun, and reflects his light to places which could not have his direct rays. Then, enquired the Marchioness, will not this be a source of twilight to all the planets, without the necessity of a dense atmosphere to environ each, since that which surrounds the sun may produce the same effect for all the planets in the vortex? From the frugality of nature, I am disposed to believe she has effected the purpose by this means only. Yet, said I, in spite of this frugality, the earth would have two causes of twilight, one of which (the dense air before the sun), would be useless, and could only serve as an object of curiosity to the frequenters of the observatory: but it may be that the earth alone sends out exhalations sufficiently gross to produce twilight; and therefore a general resource has been provided for the other planets, if their evaporations are more pure and subtile. We, perhaps, of the inhabitants of all the worlds in our vortex breathe the grossest air; did the people of the other planets know that, with what contempt they would survey us!