the observer." As a matter of fact, the envelope streaming backward from the nucleus forms a hollow tube, the opposite sides of which exhibit the same mutual repulsion as both exhibit towards the sun; hence the phenomenon would be similar to that exhibited by blowing into a closed bag of porous material covered with wisps of cotton, for example, and the gases, in addition to their rush backward from the sun, would also exhibit a radial rush outward from the longitudinal axis of the tail. This is what we actually observe, and sufficiently accounts for the phenomenon, be it altogether or only partially real, and not merely, as that author thinks it may be, apparent. It is said, in the same article, that "Bredichen has shown that where there are three tails to a comet their forms correspond with the theory that the envelopes raised from the head are principally formed of hydrogen, carbon, and iron; but this ... seems open at present to considerable doubt." At all events, these separate tails are self-repulsive, or they would be merged into each other by the sun's repulsive energy; in fact, they occupy the resultant of the direction produced by the line of the sun's repulsion and those of their own mutually repellent force,--that is to say, radial or divergent. It must not be supposed that these tails are of insignificant proportions. "When we see the tail of a comet occupying a volume thousands of times greater than that of the sun itself, the question naturally suggests itself, 'How does it happen that so vast a body can sweep through the solar system without deranging the motion of every planet?' Conceding even an extreme tenuity to the substance composing so vast a volume, one would still expect its mass to be tremendous. For instance, if we supposed the whole mass of the tail of the comet of 1843 to consist of hydrogen gas (the lightest substance known to us), yet even then the mass of the tail would have largely exceeded that of the sun. Every planet would have been dragged from its orbit by so vast a mass passing so near. We know, on the contrary, that no such effects were produced. The length of our year did not change by a single second.... Thus we are forced to admit that the actual substance of the comet was inconceivably rare.... From what we have already seen, it will be manifest that the formation of comets' tails is a process of a very marvellous nature, apparently involving forces other than those with which we are acquainted. The tail, ninety million miles in length, which was seen stretching from the head of Newton's comet nearly along the path which the retreating comet had to traverse, must, it would seem, have been formed by some force far more active than the force of gravity. The distance traversed by the comet in the last four weeks of its approach to the sun under gravity was no greater than that over which the matter of the tail, seen after the comet had circled around the sun, had been carried in a few hours.