A similar case to this occurred with respect to Biela's comet, which was to return in 1832. In calculating its reappearance in this year, Damoiseau found that it would pass through the plane of the earth's orbit on the 29th of October. Rushing away with this, the papers made out that a collision was inevitable, and the end of the world was come. But no one thought to inquire where the earth would be when the comet passed through the plane in which it revolved. Arago, however, set people's minds at rest by pointing out that at that time the earth would be a month's journey from the spot, which with the rate at which the earth is moving would correspond to a distance of sixty millions of miles. This, like other frights, passed away, but was repeated again in 1840 and 1857 with like results, and even in 1872 a similar end to the world was announced to the public for the 12th of August, on the supposed authority of a Professor at Geneva, but who had never said what was supposed. But in reality all cause of fear has now passed away, since it has been proved that the comet is made of gaseous matter in a state of extreme tenuity, so that, though it may make great show in the heavens, the whole mass may not weigh more than a few pounds; and we have in addition the testimony of experience, which might have been relied on on the occasions above referred to, for in 1770 Lexele's comet was seen to pass through the satellites of Jupiter without deranging them in the least, but was itself thrown entirely out of its path, while there is reason to believe that on the 29th of June, 1861, the earth remained several hours in the tail of a comet without having experienced the slightest inconvenience. As to the nature of comets, the opinions that have been held have been mostly very vague. Metrodorus thought they were reflections from the sun; Democritus, a concourse of several stars; Aristotle, a collection of exhalations which had become dry and inflamed; Strabo, that they are the splendour of a star enveloped in a cloud; Heracletes of Pontus, an elevated cloud which gave out much light; Epigenes, some terrestrial matter that had caught fire, and was agitated by the wind; Boecius, part of the air, coloured; Anaxagoras, sparks fallen from the elementary fire; Xenophanes, a motion and spreading out of clouds which caught fire; and Descartes, the debris of vortices that had been destroyed, the fragments of which were coming towards us. It is said that the Chaldaeans held the opinion that they were analogous to planets by their regular course, and that when we ceased to see them, it was because they had gone too far from us; and Seneca followed this explanation, since he regarded them as globes turning in the heavens, and which appear and disappear in certain times, and whose periodical motions might be known by regular observation.