Possibility of our globe passing through the tail of a comet–Has such an event ever taken place?–The great comet of 1861–Relative positions of the earth and one of the two tails of that comet–Memoir of M. Liais and the observations of Mr. Hind.

Thus far, in treating of the possibility of a rencontre between a comet and the earth, we have more especially had in view the nucleus, or rather that portion of the comet's nebulosity which constitutes the coma. The effects of the rencontre have been studied on certain hypotheses respecting the mass and physical constitution of the comet whose nucleus we have supposed to be solid; this is far from certain, and, in any case, seems to be exceptional, as it is only in certain comets that the head is sufficiently condensed to exhibit a luminous nucleus.

A rencontre, of much greater probability, is that which would arise from the passage of the earth through the voluminous nebulosity of which the tail is formed. In all probability the masses of these appendages are all but inappreciable. Whatever opinion we may form of their nature, whether we regard them with Cardan and certain savants of our day as purely optical effects without material reality, or see in them the most tenuous portions of the atmosphere of the comet projected by a repulsive force, it appears certain that they consist of quantities of matter of extremely slight mass, and of even less density.